 Okay, the purpose of this video is to give you background information on this week's lab. This week's lab deals with acids, bases, concentration, in particular concentration of acids, and we are going to pretend that we are doing a procedure called an acid-base titration. So that's where this word is coming in. We've already had background information on acids and bases. We know that acids and bases neutralize each other. What I mean by that is if you have an acid, it's relatively toxic. It will burn your skin, let's say. If you have a base, bases are also relatively toxic, they burn your skin, but if you mix them in the right amounts, you make materials that are relatively tame. That's what I mean by neutralize each other. Both of these things are toxic individually, but if you mix them in the correct amounts, they end up making something that is not necessarily toxic. This equation up here just gives you an example of this. This is an acid, HCl, that's hydrochloric acid. This stuff here is a base. It's called sodium hydroxide. This is pretty toxic if you have a lot of it around. This is also pretty toxic. If you mix them together in the correct amounts, you make salty water, which you probably know is not too toxic, and that's what I mean by neutralization. So if I gave you a liquid, she said, here's some liquid in a container, and I'm telling you that there's acid dissolved in it, but I wanted to know the concentration of the acid in the liquid. In the real world, what you would do, what you might do, if you're familiar with this type of thing, there are things called pH strips. If you have a pool, you probably have used pH strips to measure the acidity or the acid concentration in your pool water. And pH strips are just these little thin strips that are about the size of a cigarette, and there are little markers on them, and you dip the strip into your liquid, and the markers change colors, and then based on what colors they change into, you can tell what the pH of your liquid is, and remember, pH is just a fancy way of measuring acid concentration in units of molarity, except you take the logarithm of the molarity concentration and you multiply that by negative one, but basically what pH is, is measuring acid concentration. That's what you would do in the real world if I gave you a liquid and asked you how acidic it is. We are not going to pretend that we are in the real world. We're going to do it the hard way. And again, just to give you an example, again, this is hydrochloric acid mixed with sodium hydroxide, so this is the acid, this is the base. What I will tell you, and you can pause the video and work through this on your own, but this equation, as it stands right now, is balanced. What that means is there's a one in front of the HCl. There's also a one in front of the NaOH. Because this equation is balanced, what this means is, if I have one HCl molecule, I need one NaOH to neutralize it. If I have a million HCl molecules, well, I need a million NaOH molecules to neutralize it. You can pause the video and think about this, but what if I have 10 billion HCl molecules? How many NaOH molecules do I need to neutralize it? Well, on pausing the video, it's the same number, because according to this balanced equation, you need equal amounts of these things to end up making salty water. I want you to keep that in the back of your mind because that's going to become important on the next bunch of slides. You are, obviously, you're pretending everything today, but you're not going to be working with HCl in this pretend exercise. You're going to be working with something else called acetic acid. This is the formula for acetic acid. Acetic acid is the main ingredient in vinegar, so this is the stuff that makes vinegar taste tart. If you remember, acids donate H+, that's the definition of an acid. This red H here is the H that gets donated by acetic acid as H+. The other three Hs, they don't really get donated. So here's our acid, this is the base again, it's the same base as above. You can neutralize acetic acid with NaOH, and you end up making water, and this stuff over here, which is pretty harmless as well. If you look at this equation here, and you can pause the video later and confirm this if you want, but this equation is also balanced as it is. What that means is that if you have one acetic acid molecule, you need one NaOH molecule to neutralize it. And if I have three billion acetic acid molecules, then I need three billion NaOHs to neutralize it. Just to give you a sense of why I'm saying this. Like I said, it's going to become important on the next bunch of slides. But for some acids and bases, this is not always true that you need equal numbers. For the exercises that we are doing today, you will always need equal numbers of acid and base. But I just want to let you know that isn't always true for certain acids and bases. But for us, it is true. So like I said, we are going to be working with acetic acid and vinegar in our pretend exercise. And what I'm going to do is I'm going to give you a pretend vinegar liquid, where acetic acid is dissolved in it. And I'm going to ask you to try to figure out the concentration of acetic acid in our pretend liquid. And we're going to do this the hard way. We are not going to use pH strips. To explain how we're going to do this, I need to go over two thought exercises. The first thought exercise is on the next slide. Second thought exercise is on the slide after that. Here's thought exercise number one. We are going to figure out the concentration of acid in units of molarity. So here is the formula for molarity. If you don't remember it, it's the number of moles of solute. That's the stuff you dissolved. In this case, it's going to be acid or base, depending on what we are talking about, divided by the number of liters of solution. And we are going to talk about this setup here. So here's thought exercise number one. Pretend that we have 5 milliliters of 0.25 molar hydrochloric acid. So here is 5 milliliters of 0.25 molar HCl. Now in this situation, I am telling you what the concentration of the acid is. Here it is, 0.25 molar. And our acid in this case is HCl. This is not the situation you are going to be in. Because number one, you're not going to be working with HCl. You're going to be working with acetic acid. Number two, you won't know the concentration of the acid. But this time in thought exercise one, I'm telling you what the concentration is ahead of time. And there it is. This is our container with the acid in it. First question I want to ask is, how many moles of hydrochloric acid are in those 5 milliliters? How many moles of acid are running around inside of that container? Remember, moles is not the same thing as molarity. You know what the concentration of the acid is in molarity, because I'm telling you. You know how much volume you have of the acid because I'm telling you. But I want to know the number of moles. So using this equation down here, we know two of the three numbers that go into this equation. We know this number. You can pause the video and think about where that goes. But this number here on pausing goes here, because this is a molarity. So 0.25 capital N equals. You can pause the video again and think about where this number goes. Well, this is a volume. It's milliliters. So it's going to go down here. But if you notice, the units are not correct, because for molarity, we need liters. We don't need milliliters. So we need to convert this number to liters. And I will tell you to convert this to liters. To convert milliliters to liters, you divide by 1,000. 5 divided by 1,000 is 0.005 liters. So the only number that we don't know is this top one, which I'm going to call x moles, because it's a number of moles. This is going to be x moles of acid, because we want to know how many moles of acid are running around in this liquid here. Now we have to resort to algebra to figure out x. But basically what we do is we multiply both sides by 0.005 liters. If we do this on the right side, those cancel away. And x moles of acid is going to equal 0.005 times 0.25. And if you do that, you end up with this number. There are 0.00125 moles of acid running around in my 5 milliliters of liquid. And if you don't trust this number, you can do 0.00125 moles divided by 0.005 liters, and you will end up with 0.25 molar concentration. So the answer to this first question, how many moles of HCl are in those 5 milliliters? There are 0.00125 moles of acid. According to the calculation that we just did, and that calculation is correct. Second question is, how many moles of NaOH would it take to neutralize those 5 milliliters of 0.25 molar HCl? You can pause the video and think about that. On pausing the video, how many moles of NaOH would it take to neutralize? Well, if you remember from the previous slide, we said that you need equal numbers of HCl and NaOHs to neutralize each other. So if there are 0.00125 moles of acid in that liquid, then we would need 0.00125 moles of NaOH, which is the base to neutralize, our 5 milliliters of acid. So that is thought exercise number one. That is trying to do two things. It's trying to show you how to calculate moles if you know volume and concentration. And it is also trying to emphasize that in our equations, we are going to need equal numbers of moles of acid and base to neutralize each other. So now let's do thought exercise two. Thought exercise two is the situation that you are going to pretend that you were in in the lab exercise. What I mean by that is, imagine now that we have 5 milliliters of hydrochloric acid in our container. But this time, we don't know what the concentration is. That's the situation you're going to be in. How do we figure out what the concentration of the acid is in the liquid in that container? This time, we have a tool at our disposal. We have a huge supply. Let's pretend that we have a huge supply of base. And this time, we know what the concentration of the base is. So I have this big container of NaOH dissolved in water. And in my pretend example, we know what the concentration is. It's 0.2 molar base. And what I can do in my pretend exercise is I can take little drifts and drops of this and I can throw it into the acid. And I can keep throwing it in until the acid gets neutralized. And I can measure how many drifts and drops I had to throw in to the acid in order to neutralize it. And in my pretend story, let's pretend that it takes 20 milliliters. So I had to take a certain number of drifts and drops of the base to neutralize. Here's the acid over here. And if I add up all the drifts and drops, it adds up to 20 milliliters of base, of 0.2 molar base, to neutralize our 5 milliliters of acid. First question is, how many moles of base did I use to neutralize the acid? Well, remember, molarity is moles divided by liters. This thing here, that's molarity. So that's going to go on the left side of the equation. Do we know how many liters of base we needed to neutralize the acid? Well, we know how many milliliters, because it says right here, we can convert this to liters because we want liters in the molarity equation. To convert milliliters to liters, we divide by 1,000. 20 divided by 1,000 is going to be 0.02 liters. So now what we have is we know what the concentration of base is. It's 0.2 M. Capital M is molar. We know what the volume of base that we needed to neutralize was. That's 0.02 liters, because 20 milliliters is 0.02 liters. The only thing we don't know is the top number. In this case, that's x moles of base, because we're talking about a base here. So how many moles of base did I use to neutralize the acid? Well, let me clean this up. 0.2 M equals x moles of base divided by 0.02 liters. We want to figure out what x is. That means we multiply both sides by 0.02 liters. The 0.02 liters cancel on the right side. We have x moles of base is equal to 0.02 times 0.2. And if we do this multiplication out, we end up with x moles of base equals 0.004 moles. Because 0.02 times 0.2 is 0.004. So the answer to this first question is it took 0.004 moles of NaOH to neutralize our 5 milliliters of acid. Next question is, well, how many moles of acid, how many moles of HCl were actually running around in my 5 milliliters of acid in this container before it got neutralized? You can pause the video and think about that. On pausing the video, if you remember, it takes equal numbers of NaOH and HCl to neutralize each other. So if I needed 0.004 moles of NaOH to neutralize the acid, there must have been 0.004 moles of HCl in my 5 milliliters of acid solution. Again, we need equal numbers. So there must have been 0.004 moles of HCl in this container, in that liquid at the beginning before it got neutralized. So how does this help us figure out what the concentration of acid was in this container? Well, remember, molarity is moles of stuff dissolved divided by liters of solution. Well, how many moles of stuff, how many moles of acid was dissolved in there? 0.004. So that number goes there. How many liters of acid was there at the beginning? Well, it was 5 milliliters, because we told you there were 5 milliliters. And what is this in liters? Divide by 1,000, and that is this in liters is 0.005. So the concentration of acid at the beginning before anything got neutralized was 0.004 moles of acid divided by 0.005 liters of solution. 0.004 divided by 0.005 is 0.8, capital M for molarity. So in this pretend exercise, we did not know what the concentration of acid was at the beginning. We knew that we had 5 milliliters of acid. So we knew the bottom number in the molarity equation. The only thing we needed was the top number. The way that we figured out the top number is we took a big supply of base. We added little drips and drops of it to the acid until it got neutralized. We measured what volume of base we needed to neutralize the acid. We used this information and the concentration of the base to figure out how many moles of base we needed to neutralize the acid. We said, hey, if it took this many moles of base to neutralize the acid, it must have been the same number of moles of acid in the container. And if there's this many moles of acid in the container and it was originally in 0.005 liters, well, we can figure out the concentration of the acid at the beginning. It's 0.8 molar. And that's essentially the exercise that you are going to do today. The procedure that I just described is an example of something called the titration. Titration means using something that you know the concentration of or using something that you know the amount of to figure out the amount or concentration of something else that you don't know. There are many different kinds of titrations. The one that we are doing today is called an acid-base titration. The titrations are used in a lot of different disciplines, including measuring the amount of poisons that are on hand, how strong a vaccine is, et cetera, et cetera. We are doing an acid-base titration. And just to come back here, I said titration is when you figure out the concentration of something by using something else that you already know the concentration of. In thought exercise two, we already knew the concentration of base. We said it was 0.2 molar. And we did not know the concentration of acid. And we used this stuff here and threw little bits of it into the acid until it got neutralized and then used the information to figure out the concentration of acid. So that's what an acid-base titration is. Another thing that I want to point out, in the real procedure that you would have done, you would have had a flask like this with acid in it. You would have used this thing here, which kind of looks like a big syringe. This thing is called a burette. All it really is is a fancy graduated cylinder that lets you add liquids like the base to other liquids, like this acid, in little drips and drops. So there's a little valve down here that lets liquid fall into whatever is underneath. So here's our burette. There's a little valve here. We throw the base into the burette. So there's a whole bunch of NaOH in the burette. There's acid underneath. We open up the valve and we let the base run into the acid until it gets neutralized. That's the actual procedure that you would have done. And then there are little markings on the burette that let you figure out very precisely how much base you actually have to throw into this container to neutralize the acid. So this burette thing is just a really fancy way of measuring how much volume you deliver into whatever is waiting below. In this week's lab exercise, what you're going to do is you're going to pretend that you're doing an acid-based titration to figure out the concentration of acid in grocery store vinegar, in stop-and-shop vinegar. And remember, the acid in stop-and-shop vinegar is acetic acid. And there's the formula for acetic acid. OK, so here's a brief description of what you're going to pretend you're going to do. You're going to pretend that you take five milliliters of stop-and-shop vinegar, which has acetic acid in it. You don't know what the concentration is in molarity. You're going to add a certain number of drops of sodium hydroxide. So this is the base that's going to neutralize the acetic acid. You're going to add a certain number of drops of sodium hydroxide to the acid until it gets neutralized. And the lab exercise itself tells you what the concentration of NaOH is. In real life, if you were going to do this in person, it requires adding the NaOH in tiny little steps, which is why you have the burette. This is me trying to draw a burette. And there's a little valve here. So the setup would have been you have five milliliters of acid. You have base in the burette. And then you open up the valve, and you let the base flow into the acid until it gets neutralized. And then you measure using the little markings on the burette, measure how much base you actually threw into the acid to neutralize it. Then once the acetic acid is neutralized, you can figure out how many moles of NaOH you needed to neutralize the acid, because you'll know what the concentration of base is. You'll know by looking at the burette how much volume you needed. And then you can do the rest of the calculations that we did in thought exercise 2. And that's how you figure out the concentration of acid. So here's the setup again. You're going to have five milliliters of acetic acid. You're going to have a burette that has NaOH with a certain concentration. I don't remember what the exact concentration is in the pretend lab exercise. You're going to add the base into the acid until the acid gets neutralized, and then you do the calculations. At this point, you should be asking yourself an important question, and I'm sure many of you are. And that is, how do you know when the acid got neutralized? The acid does not jump up and tell you I got neutralized. It's colorless, it's clear. As far as you can tell, unless you taste it, it's going to look just like water. There's a trick that you can do. And that is, there is a material that you're going to pretend that you're using, and you would have used this in real life, called phenolphthalein, it has a weird spelling. What you're going to do is, this is the acid, and you're going to add, you would pretend to add one or two drops of phenolphthalein into the acid liquid before anything happened, before you added any base into the acid. Phenolphthalein is a special chemical. If it is mixed in with an acidic liquid, it is colorless. If it is mixed in with a basic liquid, it is bright pink. And what you would do, what you would pretend to do, is you would add a couple of drops of phenolphthalein into the acid, it would look like nothing happened, the liquid would still be colorless. And then you would add drips and drops of the base into the acid, until this liquid down here turned faint pink. If it turns bright pink, that means it's in a basic liquid, which means you added too much base. But if it turns faint pink, that means you're teetering right on the edge between an acidic liquid and a basic liquid, which means you are essentially neutral. So what you would have done, is you would have added drips and drops of the base into the acid until this stuff turned faint, faint pink. And that would mean, okay, the acid got neutralized, now I can stop the experiment, measure how much volume of base I had to throw into the liquid to neutralize it, and then do the calculations. So a little bit more detail on what you would have done, you would have taken five milliliters of acetic acid in a little container, there's our acid, you would have put it in an Erlenmeyer flask, that's a flask that roughly looks like that horrible drawing. You would have added one to two drops of phenolphthalein into the liquid, because it's initially mixed in with an acidic liquid, it would still be colorless, it'd swirl it around, and now you use this and you throw base into that liquid until it turns faint, faint pink. This is just to show you what it looks like for real. This is a liquid that has phenolphthalein mixed in with it, but it's acidic and you can see it's colorless. This is basically neutral, you can maybe see a faint, faint pink color there, that's kind of the color that you're looking for, this is still neutralish. And over here, this is too far as far as neutral is concerned. These liquids are almost certainly basic, they are above pH seven. So if you did this in an on-ground class, a lot of times the students overshoot and they end up with liquids that are much too bright pink and then I yell at them and tell them to do it again. So what you're gonna do is you're gonna pretend that you did this experiment, figure out the concentration of acetic acid using the approach that I described in thought exercise two. In real life, this is tedious. Students think it's really cool to see the pink color in the liquid come and go, but after a while they get bored with it and it's really tedious and it's very easy to blow past the neutralization point and end up with a bright pink liquid and have to do it again. You have the luxury of not having to do it again because you're just gonna look at a picture and figure out the concentrations that way. And this is a little bit of the lab. So here's the setup and I am showing you, here's your liquid and it's five millimeters of stop and shop vinegar which has a acetic acid in it. Here is your big supply of base, the NaOH and I'm telling you what the concentration is. There it is. Then here is the burette. The base is in there. See, there's the base in the burette. The burette, burettes are a little bit weird but this is done for your convenience. The top of the burette starts at zero and the bottom of the burette ends at a higher number. So if you set up the base into the burette where the amount of base starts at the zero line then it's easy to read how much volume you delivered into the acid. So here's our acid before it gets neutralized. Here's our acid after it got neutralized. See how it's faint pink and then you can make a measurement. You're gonna have to look at this and figure out what volume of base you needed to deliver into the acid to neutralize it. So you're gonna have to read these little markings here and make an estimate for how much volume of base you delivered. Then you're gonna fill in this data table and answer these three questions. And that's it. So good luck.