He's formula starts with h. And you might recall last time there was a that we're talking about ionic compounds. Can I enter the positive ion first and the anion or negative ion last. So this suggests that each is going to be a positive ion. For essence. Here are some examples. Let's HCl. Anyway. Hydrochloric acid we know the name of this one. Nitric acid is correct. At this one Acid How about this one phosphoric acid is correct. But the point is what all of these have in common formulas that start with age and they all turn out to be good sources of H plus ions. They can define or it would be an example of a base. Plate. That's right. Yes. Ammonia is in fact the Bates Yes. So does tend to be on the basics Anyway define what obeisance. Ok there are other examples. Again none of which happened to be one of the things you mentioned my slide. But we'll come back to things like that a little bit later on. Among the things you can say about some basic certainly have formulas that end with an OH. Last time we talked about the polyatomic ion known as the hydroxide ion. That's coming at the end of the formula that means that OH minus ions are generally found in bases. Here are a few other examples of the ones that were mentioned today by bus Nih is sodium hydroxide sometimes known as why KOH potassium hydroxide calcium hydroxide Ca OH parentheses to sometimes used to adjust the acidity of soils. Now I'm using these particular example reasons because what happens when you mix an acid and a base together yes one product that's typically formed is water and without water it comes from is H plus ion from the acid getting together with OH minus ions from the base. What else his foreign besides one Yeah some ionic compounds maybe we'll just say sol. Chemists use those terms pretty much interchangeable but the point is if you mix together an acid and a base and the correct proportions they neutralize ego which means that a chemical reaction takes place and the new substances that are formed are neither acids or bases. Classic example. Hydrochloric acid HCl reacting with sodium hydroxide to form water and the by-product of this place would be table salt sodium chloride. So get mixed together HCl and NAOH and the rank of 4a because you're going to get saltwater This notation is commonly used to represent a chemical reaction. The things on the left side are the compounds that were there before the reaction took place. Those are called the reactants. The things on the right side the new substances that are formed as a result of the chemical reaction are called the products. The arrow in the middle it's sort of like the verb in a sentence. You can read that as either producers or reacts to form anything that suggests that a chemical reaction takes place and new substances are being formed. So for example if I were to read this out loud And I say that hydrochloric acid and sodium hydroxide reactor for water and sodium chloride. Does this make sense all right. Next question. Is this a balanced chemical equation and what do we mean by that the answer is yes. It is balanced how could you tell us okay in other words if you think back to what John Dalton What happens during a chemical reaction we simply rearrange the atoms to make new compounds. We don't gain or lose any atoms create or destroy any map. If you look at this equation you can see that there's one chlorine atom among the reactants M1 among the class and one sodium atom and the products and one sodium atom in the reactants. And what oxygen on the left side one oxygen on the right side right side has two hydrogen atoms. So does the left side one of them is over here and the other one is over here. So the point is there's the same number of atoms of each element on both sides of the arrow That's the case that this isn't a balanced equation. But anybody like more time with this slide alright so the point is the definition of a balanced equation. It's an equation that has the same number of atoms of each element both sides of the arrow. This equation is not balanced. What would we have to do to balance yes say it Or another chlorine. So in other words what you're telling me if you want to change Cl two Cl three right there's no such thing as CL three. There's no such thing as a molecule consisting only of three chlorine atoms. Yes. Yes. Okay what you are allowed to do is adjust the coefficients in front of these things. Specifically what coefficients what do you have to put in to make this whole thing a balanced equation yes. Right but a two here a three here and a 2i. And bend your head The equation now in your textbook you might see some illustrations that look something like this. And the point is if you want to draw an illustration like this for what's going on. When the chemical reaction takes place you can use any little symbols you make up to represent the atoms. What I'm doing here is representing iron atoms by red squares. Can chlorine atoms by reinsert enters the point is a molecule of CO2 consists of two green circles stuck together. And then the product f BCL3 consists of one iron atom one red square and three green circles stuck together like that for the three coordinates. But the point is that we're representing and atoms by red squares imploring atoms by green circles. And if you look at what happens when you write out the illustrations were this balanced equation you can see that there's two red squares and six green circles both on the left side and on the right side. So all we've done is rearrange the atoms and this is a balanced equation. Now you can see that without the illustrations that's perfectly fine. But if drawing the illustrations helps you to see what's going on by all means and draw the illustrations and the textbook will probably do the same sort of thing. Makes sense. Let's look at another example What would you have to do to balance this equation which represents hydrogen and oxygen reacting to form. Yes to from here to here to here. Once we do that two times 24 hydrogens on the left side two times 24 hydrogens on the right side two times 12 oxygens on the right side also two oxygens on the left side. Any coefficient multiplies It follows that. So for example the two here in front of the F BCL3 volatilize iron by two and Cl three by two for a total of 614 it makes sense. Here's the illustration for this one. And if the little water molecules drawn here sort of looks a little bit like a Mickey Mouse hat. We'll find out later that that's actually not a terrible representation of what a water molecule actually does look like. We'll talk later on about the geometry of molecules like this. Is this a balanced equation I heard some people say no. Yes. That's right. In other words this is in fact a balanced equation but it is not the equation of hydrogen reacting with oxygen to form one. Because this is not the formula for water. Does anybody know what compound this is the formula yes say again. There's a more common name for it. Anyway now yep. You're closer One for oxide hydrogen peroxide is what I was like. Well I'll get back to your question in just a moment. What hydrogen peroxide is used for you can put it on a cut to disinfect. It. Sometimes uses a dot for each point is it's not good for quenching your thirst. So what you're trying to do is talk about hydrogen and oxygen reacting to form water. That is not the right answers and what is in general you are not allowed to change formulas but you are allowed to put coefficients in front of the formulas to describe what's going on with your question. So I will show you how Be able to make wire. That's totally different situation. Although it turns out that if you simply take hydrogen gas and oxygen gas and mix them together and provide a little bit of energy. You generally get water not hydrogen peroxide the Harvard question for the hard thing to do is actually make the hydrogen peroxide but we'll talk more about that maybe later on like worse for you and I talked about that's anybody more time at this line. Everybody comfortable with that concept. Right in that case I want to have you try a couple of these at your desk as before you can work together or individually but see you didn't write balanced equations for these two which are not balance. The first equation involves the reaction of aluminum chloride with water to form aluminum oxide and hydrochloric acid. The second is the combustion of a hybrid orbital benzene C6H6. When it reacts with oxygen the products are carbon dioxide and one. Right now neither of those is a balanced equation. C we're going to insert the appropriate coefficients to make them balanced equations And you may work together or individually as you see fit. Yeah Yeah yeah. Well here's an answer. They like first one. Yes. Three. Wire. Nothing there. Okay so earth coefficients there to free 16. Ok to solve something like this you pretty much just kind of have to start someplace and you would you have to do for example you can see that there's two aluminums right here. So we need two aluminums. I'm putting a two in front the A-LC L3 fixes that problem. That to also multiplies the chlorines So that gives you six fluorines on the left side. That's why six HCl. On the right side you gave yourself six points that multiplies hydrogen by six which is why I need three waters and that also balances the oxygens are the same. For the second one yes to that team. Well okay. Now this one might have been a little bit more challenging. Because when you first look at these you could say well okay six carbon atoms. So multiply this by 66 hydrogen atoms multiply this by three. The problem is if you do that that gives you 15 oxygen And thought well you could solve the problem by putting a fractional coefficient like 15 over two in front of the oxygen that is acceptable but a lot of people don't like fractional coefficients. You can get around that problem by realizing one thing. Any multiple of the balanced equation is still a balanced equation. If you had 15 over two here you don't like that. Just multiply everything by two. And if you do that'll give me 215 well six which is the answer I got. If you got some aids are different from these for example for six to 12. That's still a balanced equation. Because if you multiply all the coefficients by saying Are you still keep your balanced equation the concepts make sense. Okay there will be an additional problems at the end of chapter four in your textbook. For more practice in writing balanced equations. Anybody need a little more time on this slide. Okay is is limited technique. Science. I thought I just told you what the technique was that organized. Well okay some people like to keep track of what's going on by making like little scoreboard for themselves. Carbon-hydrogen Decide normal nitrogen oxygen on this side then all the atoms. You can do that. That's a good way to check yourself. And any other questions about this slide and if they want more time with this one. Okay at this point it will back up to chapter two in specific ways. Section 2.7. Chemical nomenclature. Chemical nomenclature has to do with how compounds get their names Now earlier I said that you should be practicing learning the names and symbols of elements one through 36 on the periodic table. Have you been practicing gangs because what I'm about to say it's not going to make any sense at all that you can't match names and symbols of elements. Having said that there are different kinds of compounds. For example some compounds are made of ions will start there. But let me point out something about how you tell whether or not a Kanban is made of Life You can look at page 95 in your textbook as a periodic table there. And it might be hard to see on screen but there's some color coding here. Most of the elements appear in yellow. The ones on the right side appearing green hydrogen also appears in green. And there's some on the borderline between the yellow agree they're shaded in purple. The yellow ones are metals. The green ones are the ones on the borderline at group or sometimes called metalloids or the metalloids tend to behave more like the nonmetals. Wonder Off that a compound is made of islands is if it contains at least one metal and at least one nonmetal. So you could use the periodic table to help figure this that the metals in general are going to be the positive ions or the canyons. The negative ions the anions are going to be the non-metals for the most part. And again URL to name an ionic compound we name the canines simply using the name of the element itself and then name the anion we attach the suffix ID. Let's look at a few examples and I think this will make more sense. Name this combat sodium chloride of course cable. So but the point is in a sodium Cl is chlorine. Chlorine becomes chloride. What's it becomes part of our combat name. This cockpit. Potassium bromide K is potassium. Bromine becomes worldwide. Is this making sense okay CaO calcium oxide also known as lime used to adjust the acidity of soil. Try this compound. Lithium is sulfide is absolutely correct. You don't need to say die sulfide or anything like that. Because lithium will always form plus one ions. Sulfur forms minus two ions. At the point of any compound is and the positive charges and the negative charges have to cancel each other. So these two must combine and this ratio there's no other combat for lithium reacts with sulfur. How about this one let us there's a day. That's better Museum iodide is correct. Again no need for further explanation. Magnesium forms plus two ions iodine forms a minus one i and that must be the ratio between those two. So all you need is magnesium iodide. Pry this last one. Aluminum fluoride is correct. Aluminum forms a plus three ion. Soviet 83 minus1 fluoride ions cancel out. Aluminum fluoride is the correct name that got any questions about any of these exams straightforward it up. Okay Where it gets a little bit more complicated is when the metals come from the middle part of the periodic table. A little bit later on in your textbook on page 99. They show you a different version of the periodic table which shows that for the most part elements to the same columns form the same kinds of ions. Everything in this column forms a plus one ion everything in this column 4s plus DY over here negative one and this column negative two in this column and so on. It's generally pretty straightforward except for these guys at the middle where you can see that there are several different possibilities So those guys in the middle are referred to as the transition metals. And there are numerous possibilities for the different transition metals. For example copper can form either a positive one ion or a positive two I hire can form Positive two positive three I and some of the transition metals get even more complicated than that manganese. For example I'm going for a positive seven positive for positive two. There are several possibilities for chromium. Invariants. What must be the charge on copper in this cop out plus what we know that chlorine is typically minus one. If the compound is going to be neutral this copper must be plus one. Well it must be the charge on copper in this gaba. Plus two here we have two fluorines minus one h. So for this to be neutral copper here has to be plus two. In general to ways of dealing with transition metal nomenclature. The most common these days is to use a Roman numeral to refer to the charge. Since copper here forms a positive one ion. We include the Roman numeral one after copper. And this compound is called copper one chloride. By similar reasoning how would you date this copper two chloride with a Roman numeral. There's an older system that uses the suffixes egg and us with it being the larger charge and us being the smaller charge. And the only problem with that system as if you're going to use that system the base word has to come from the Latin name of the element. Symbol C u for copper comes from its Latin name **** room. That's where the term coop Russ fluoride comes from for an alternative name for this campaign. What would be an alternative name for this compound using that system Kubrick chloride is correct except this means the larger charge Plus suffix means the smaller challenge. This com founded by any reasonable method. Iron iron one. Yes iron oxide oxygen forms a minus 2i. So in this case ion must be positive two. And you could use the Roman numeral II or two oxide to name that. Try naming this compound in a similar way but realize that the Roman numeral prefers to individual ions and their charges Iron three oxide is correct. If each oxygen is minus two then three of them would be minus six. That means the two wires between them must be plus six which means each one individually must be plus three. Alternative possibility. The symbol FV comes with a Latin name pheromone for iron and if this is ferrous oxide What's this by a similar nomenclature ferric oxide is correct. Well you're going to be taking a multiple choice. However the Roman numeral system is much more common that the x and the yeses these dates. Here's the one thing I will admonish. You will then either use the Roman numeral or use the x and the S's but don't combine the two. Although I did want to see a bottle of this compound on a shelf once with a label that said iron oxide. However there is no such thing as ironic oxide. It's either iron three oxide or ferric oxide but not this question. So if you were personally singular us Now I'm not saying singular or plural I'm saying larger charge and smaller term. But what in other words iron is plus three so it gets the iron plus two gives us what we will get back to that later today. For now any other questions about this slide anybody need more time with this slides. Yes. I would focus mostly on any other questions. Yes. No but you'll be provided with a periodic table when you came to dig in. And he looked at the old ripeness exam yet. If you do you will not only see the practice exam. You will see exactly what the periodic table will provide you with. Any other questions about this slide. Omega. Let's consider compounds made of molecules. Ionic compounds Have at least one mental and at least one. But compounds made of molecules tend to be made only of non-metals one shaded in green and that periodic table we showed you a few. Even though there are no ions in the air we can still name the combat as though there were a cat ion and an anion we still use the suffix IDE for whatever comes last in the name. But now we use prefixes to tell us how many of each atom we happened to have again and we look at a few examples. I think this will make more sense And this got banned carbon dioxide of course C is carbon O is oxygen IDE suffix. And the two is represented by the die in front of the except what's the prefix that means one. Motto. We don't generally use the prefix mono. For example we don't say motto carbon dioxide because we're mentioning the element at all. You can be pretty sure there's at least one of them. And the foreigner with one notable exception. What's this got mapped on carbon monoxide. Here we have the model because carbon and oxygen can make two different compounds when they get together. And if you just say carbon oxide it would be a little bit ambiguous as to which one you mean. So we call desktop app carbon monoxide but other than that we don't use mono burial. Name this compound sulphur flora the prefix being six. So SF6 sulfur hexafluoride about N2O5. Say and you're almost there. Yeah we have a few nitrogens. So I rather that dinitrogen pentoxide Some people say pad oxide. So I believe that that just to make it easier to pronounce either way is okay. How about P40 ten tetra phosphorous. Decca oxide is correct. Deca means ten. Tetra means for me. Is this making sense right Vb R3. One fact boron tribe worldwide is correct. Me as Boron BR as bromine. Three is brought about that last guy. Even chemists call it water water. It has been around for a long time. People were calling water water before somebody thought of in a systematic way of naming compounds and even chemists who pretty much pull it all up. I gave out this handout today. And I will pull your attention to All right. Let me just read a little bit of this article to me. It says here that a fellow named William turbo around Lake Illinois found on the Fox News Network website a story that said that officials of elisa DAO California. We're thinking of banning plastic foam cups because the cups were made with dihydrogen monoxide goof occurred when I'm Lisa of ego paralegals believed an internet host site. It describe dihydrogen monoxide as an odorless tasteless chemical that is lethal if inhale Every word of that of course is true. So it'll never wind up as snorting water rollover to believe what you read on the internet with a monochrome of sodium fluoride is your judgment. And main point is your ally Article 101. Although if you want to have more fun with dihydrogen monoxide it has its own website. Dh m.org is a website maintained by somebody right near and you aren't Delaware. Is dedicated to your work. Dihydrogen monoxide How do you acids get their names acids have formulas that begin but having said that there's more than one Binary acids are made of a grand total of two elements hydrogen and something else which we'll call x. In general to name a binary acid is the prefix hydro and the suffix ic acid. And in the middle you get some variation of the name of element. Named this. Hydrochloric acid is correct. Ceo is chlorine so we stick polar in the middle between hydro And this combat I grow roaming acid turns out to be right VR as Romain. So I wrote bromine gas. Try this way. They don't need to die because again the only possible ratio of hydrogen and sulfur is two to one. You do need the hydro. Hydro hydro sulfuric acid is diet. They just eat the whole name of silver debug acid suffixes. Okay there's a few examples of binary acids The other kinds of acids are called oxyacids because in addition to hydrogen at X they also contain oxygen. Oxyacids we don't use hydro. But we do use a particular suffix that tells you something about how many oxygen atoms are there. At this time there is no Roman numeral system. You have no choice but to use the x. And the US is in means larger number of oxygens. Us feeds a smaller number of oxygens. Will show you some examples in just a few moments where people are done with this line Water acidic will actually address that question later on this semester when we talk more about the acid-base properties. So I'm going to table that question for probably about two months. We'll get back to that. They're getting what they need from this slide. Okay let's look at some examples of how oxy acids get their names. I told you the name of this combat earlier. Let's see it was Dutch and what's his goal nitric acid is correct. So if HNO3 is nitric acid To nitrous acid is correct. Larger number lot to unions gets the suffix. Smaller number of oxygens gets the US suffering's named this cockpit. Sulfuric acid is correct. So what's this combat sulfuric acid again a larger number of oxygens it smaller number of oxygens. Let's. Here's about as bad as it gets. There are four different oxy acids based on chlorine. If I tell you flat out that HClO Three is called core Rick What would you think HClO to would be call more or less correct what about the others well it means more thus means less. What do we do about less than less well there's a prefix that you may have encountered in biology class. The prefix hypo means not enough. So we use hypo in this context to me even fewer oxygens than the US situation. But the prefix hypo in front of chlorous acid HClO is called hypo chlorous acid. What's the opposite of hypo height per se you would think logically that this would be called hyper chloric acid and it almost is except for some reason they decided to leave off the HY and it's just called per core is nasser. I don't make the rules. I just tell you what the. Here's one thing that actually is kind of system that we showed you before that elements in the same column on the periodic table will behave the same way it chemically. They also get named in similar ways. Chlorine bromine and iodine All the same column on the periodic table. So if this is perchloric acid what's this per roaming acid is correct. And what's this somebody pretty close there per i yielding acid is absolutely right. This is a periodic table. This is per IoT acid. Spelled the same pronounced differently. Fortunately it's not an oral exam if you're going to be taking so long as you can spell it. But you can tell that after you practice a whole lot of problems at the end of jacketed. Yes. Having no matter. Same question he was just asking and the answer to that question is before you take your exam you will have that a lot of practice problems at the end of chapter two. Any other questions about this slide. Let me go to the next slide. You may want to refer back to your notes for this slide. Because we're going to look at what happens when we take oxy acids and make salts out. When you read Hydrogen of an oxyacid with something else you make a salt out of it. Today the saw that depends on what the name of the original acid once. If the suffix from the original acid was it acid the name of the song ends in age 80. But if the original acid had an ending US acid then the name of the song ends in ite INT. Here's some examples refer back to the previous page and your notes. If you which name this compound. Silver silver nitrate is correct. Hno3 is nitric acid when replace the H with a silver silver shows up in the name but nitric acid becomes night. Great. Name this compound. Calcium. Sulfite HQ. So three is sulfuric acid. Us acid becomes eight. Once you form an ionic salt and try this one Sodium or sodium very good. And I like how you reason that. Yep the original acid HClO was hypo floor us acid. Sodium. We keep the IPO and the HClO_4 us asset becomes height in his soul. Who knows where it the grocery one would go to buy a bottle containing a solution of sodium hypochlorite. Ion would you walk down the grocery store You might know somebody said it before but we're talking about acids and bases. Yeah so you know that you're walking down the right file. You're actually walking down a laundry. Bleach my-delay. The trade name Clorox comes from the fact that the active ingredient is a chlorine and oxygen. I'm going to tell you flat out that H2CO3 is called carbonic. So that being the case theme this compound. Sodium. Sodium carbonate is correct. It acid becomes eight. So this is sodium carbonate. Now let's think about this one to go from here to here. We replaced both hydrogens with sodium. What happens if we only replaced one hydrogen with sodium well what made an E in this Kanban is to just name it like it looks sodium hydrogen carbonate. You can do that A more common name for it is sodium by carbonate. In this case five is not being too. It means halfway. This combat is sort of halfway between this got back this compound. That's what the prefix bi means. Where the grocery store when you go to buy yourself some sodium bicarbonate. Baking aisle because that's all the baking soda by most normal people. By the way the soda baking soda comes from sodium and makes sense This takes practice but there are problems at the end of chapter two for you to practice what anyone like more time with this law. To wrap up for the day math. I trust you know the difference between qualitative analysis and quantitative analysis. Qualitative analysis The numbers it generally involves asking questions that can be answered by either yes or no. As opposed to quantitative analysis which involves asking questions like how much or how many questions that must be answered using numbers. We're going to be moving into a segment of the course that is much more quantitative in nature. So for about the next couple of weeks we're going to be doing a lot of public wrenching. However we will sooner or later get back to the more qualitative aspects of the course. For those of you who are big fans of numbers. But we're gonna talk about today is something called combustion analysis. There are well over 20 million different chemical substances that are known to exist. Many of them are composed of just three elements carbon hydrogen and oxygen. At more such compounds are being discovered every day or created it. And the point is when somebody has some new compound like this but they like to try to do is figure out what its formula is. One way of doing that is taking this compound heating it up to high temperatures and blasting it with oxygen. If you do that all of the carbon becomes carbon dioxide all the hydrogen becomes water. Both of these are gases at the high temperature of the combustion of it The point is a combustion analysis apparatus can be fitted with a couple of chambers whose purpose is to absorb the CO2 absorb the H2O. And the point is you can weigh these things before after the experiment and find out what mass of CO2 what mass of water was produced during this reaction if you measure the mass of this stuff. And if you also know its molecular weight and we mentioned before there are many different techniques that are available these days for determining molecular weights of combat. They even generally figure out what the formula of this compound is. Nevertheless be following along in the lecture notes On page 28 I outline a technique there for solving problems and combustion analysis. But on page 29 I present an alternative method for solving problems in combustion. The method on page 28 is pretty typical of what you find in most textbooks. The method on page 29 I think it's somewhat simpler and was actually suggested by one of my former students a long time. And one of the other things I like about the technique on page 29 is that it's all based on principles that you've already seen. So what I want to do for the rest of our time together today is walk you through a combustion analysis problem. And using that similar technique does anybody need more time with this line okay here is the outline for the technique that I think is the easiest way to solve combustion analysis. And all of this should look familiar. Step one law conservation of bands. One thing that is probably not given an a combustion analysis problem. What mass of oxygen was consumed during the reaction but you can use the law of conservation of mass to figure that out. We'll show you an example animal. Step two Bert all ram quantities into mole quantities. Well we know how to do that. We divide the mass of whatever it is by its molecular weight to get moles. Step three we can use these mole quantities as coefficients in the balanced equation. However most of the time we want our coefficients to be whole numbers. We had a little trip we showed you awhile back to keep numbers in the same ratio but make whole numbers out of them divide by the smallest number. That's step three. And then finally step four. Now that we know the coefficients for the balanced equation we want to write from step three. Now we have to do is write a balanced equation Except most of the time when we write a balanced equation we know what the formulas are and we change the coefficients. Here we're going to know what the coefficients are and we have to write the correct formula inserting the correct subscripts to make it a balanced equation. So shut this down and then we'll walk you through an example where hopefully it'll become more obvious what all this means. Yeah. Everybody have what they need from this slide. All right let's look at the problem that appears in the lecture notes. The same problem appears on both pages. 2829 It's just solve two different ways. Suppose that you're the scientist a long time ago who discovered the compound that we now call vitamin C. Also known as ascorbic acid. You measure its molecular weight. You find it to be a 176 grams per mole And then you do the combustion experiment. You take a sample of vitamin C with a mass of 5.1 milligrams. And by the way one of the things that's nice about combustion analysis you don't need a whole lot of the compound to be able to do it just a few milligrams is usually enough. You take 5.11 milligrams of vitamin C put it into the combustion oven heat it up to high temperatures blow oxygen through and the products that are formed or 7.66 milligrams of carbon dioxide and 2.09 milligrams of one. And the question is what are x y and z in the formula but vitamin C All right. Step one law of conservation of mass law of conservation of mass is the mass of the products should be equal to the mass of the reactants. We know what the masses of these two carbon dioxides 7.66 milligrams water 2.09 milligrams. Add those two together. We know what the mass of this is. 5.11 the only thing we know is the mass of oxygen that when we add these two together it's practice one that should leave us with the mass of oxygen Do that in this particular case we see that 4.64 milligrams of oxygen was consumed during the course of this reaction. Are there any questions about step one everybody okay all right. Step two is going to be to convert all these RAM or milligram quantities into moles. Essentially what we're going to do is divide by the molecular weights. Or the data given in the problem is that the molecular weight of vitamin C is 176 grams per mole. So we take our 5.11 milligrams of vitamin C divided by a thousand to convert milligrams to grams and then divide by a 176 grams per ball ought to give us 2.9 times ten to the negative five moles of vitamin C. Now just as an aside You could leave out this factor of 1000 year because a 176 grams per mole is the same thing as a 176 milligrams per millimole. That is you could do this problem in millivolts if you wanted to. Here's what I have found over the years. People are comfortable with things like milligrams millilitres millimeters. That's okay. Milli moles freak people out. So I'm going to do this problem in bowls. But if you want to do it in milli moles you should get the same it. The point is we now convert all of the Send them all quantities when oxygen from step one we calculated that 4.64 milligrams of oxygen. This reaction divided by 1000 to get grams divided by the molecular weight of O2 which is 32 grams per mole 1.45 times ten to the negative four moles of oxygen here. Carbon dioxide 7.66 milligrams of that divide by 1000 divide by the molecular weight of CO2 which is 44.01 grams per mole 1.74 times ten to the negative fourth. Moles of CO2. Finally water 2.09 kilograms divided by 1000 divide by the molecular weight of water at 18.02 ramp 1.16 times ten to the negative fourth moles of H2O. Now the point behind doing all of this is that we now want to use these numbers as the coefficients in the balanced equation that we're about to write except no we know because what we want is to keep these numbers in the same ratio but make whole numbers out of how do we do that Back there yeah divide by the smallest number each of these four numbers is the smallest number. The first one anything times ten to the negative five is smaller than anything times ten to the negative four. So it's step three. What we're gonna do is take all those small quantities we just calculated and divide by the smallest number which is 2.9 times ten to the negative five. We know that when we divide this one by 2.9 times five we're going to get one. And hopefully the others are going All numbers as well. And we'll do that in just a moment as soon as people are done with this line again here. All right. So here's what happens to 0.90 times e to the negative five divided by itself is one. That's going to be the coefficient for c x h y z and our chemical equation Oxygen 1.45 times the negative four moles divided by Du Bois 9-0 times negative five. Turns out to be five. That's gotta be the coefficient of NO2 and our balanced equation. Carbon dioxide. What 0.74 times then they did a four moles divided by 2.90 to 56 moles of CO2 and our balanced equation. Finally water 1.16 times negative four moles of water divided by 2.905. Turns out to be four. So for the moment Our balanced equation looks like this. Except it's really not a balanced equation at the moment because we don't know what x y and z are. But what must X B to make this a balanced equation six because we have six carbon atoms on the right side. So we need six carbon atoms on the left side. About why 84 times 28 hydrogens on the right side I need eight hydrogens on the left side. About C. Yeah yeah. I didn't quite hear you went in here the right answer and what must be the value of z. Turns out to be six. Other right-side six times 212 oxygens their **** for more oxygens there for a total of 16. So we need 16 ought to do the left side. We have ten oxygens right here. Therefore the other six must have come from the original combat. So by adding and subtracting You can arrive at the correct subscripts and therefore the molecular formula for vitamin C should be C6 age 8061 of the nice things about combustion problems is that you can always check yourself when you're done. Because the right answer should match up with the molecular weight that you were given in the problem. And if you figure out the molecular weight for C6H8O6 it should come out to be pretty close to a 176 bring any questions about anything on this slide or any aspect of this problem. Yes Well that's why I'm saying check against the molecular weight. If we don't give you the molecular weight then the best you could do is get the empirical formula. But if you know the molecular weight should be able to get them elected. I guess make it right by all don not done yet. Yes. Delivery. For the first part yes somewhere around step to you via the molecular weight divide vitamin C or whatever. So in every breath Before I let you go I have one more word that I want to say is toward that must not be utter. The word that strikes terror into the hearts of chemistry students throughout the land. You are just doing stoichiometry. Stoichiometry isn't that bad I'm back Thursday will show you. Why. I said I'm not an anxious. Well actually smell things without real. For the professor. You get bored or the point is that once you divide it by 15 years given information periodically or we waste a lot of them especially if you're used to that. He said What do you want it says yes actually I need to send out instructions for doing that. And I probably will do later this. Everybody is death brought it. Back to its old old Joyce formats. I'm looking around me. He said to me. Anyway. Yes. Thank you. Thank you realise. Realize Naomi Klein. Mm-hm.
chem103-070-20190910-153001.mp4
From Dana Chatellier September 10, 2019
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