Introduction to Titrations in Chemistry

Welcome back, Chem 100L. This is your lecture tutorial for experiment number 8, which is on titrations. A titration setup often looks like this in the picture, where there is a compound in a burette that is being added to another compound. in a beaker or flask below and you're adding compound one to compound two until you have exactly the right stoichiometric match so that both substances completely react and they form product without leaving any reactant behind. So the idea is that each reaction has some stoichiometric ratios. They react in a certain ratio that is specific to the compounds and that ratio is a little bit like a cooking recipe. So if you're making a grilled cheese sandwich, you know that you will probably use I would use two slices of bread and one slice of cheese. It's just most basic grilled cheese sandwich. And so the ratio of bread to cheese is two to one. And if I want to make grilled cheese sandwiches so that I don't have any leftover bread or any leftover cheese, that would be the same procedure I would use to do a titration where I want my starting ingredients one and two to be completely used up all turned into product. Somewhat like the bread and cheese, I want all the bread and all the cheese to be used up and all made into grilled cheese sandwiches. So that's essentially the governing principle that allows us to use titrations to understand our compounds. The goal of this experiment is to use titration to find the concentration, sometimes called the molarity, of an unknown acid or a base using a titration or volumetric techniques. And the hypothesis is that at the equivalence point where acid equals base, the amount of both are the same according to the ratio. where both is used up, the solution is now neutral. It's neither acidic nor basic and that at that point the moles are equal and if they're equal then we can use that information to tell something about the concentration of the unknown acid or the unknown base. In this experiment you will have an unknown acid. So acid-base neutralization, we're adding an acid and a base together so that they completely react with each other and leave a neutral solution. Some examples of things that are acids, so acids are usually indicated in red so I'm going to use red. Things that are acids have H plus ions and those are shown in the chemical formula as the H's and the front. on the left hand side of the chemical formula. You can have more than one H, as you can see in this last one, H2SO4, this is sulfuric acid. And it contains two H plus ions, so it can release two H plus ions. One of the reasons why sulfuric acid is so corrosive and so strong, because it's got more than one H plus to release, is not the only reason. There are weak acids that have have even more H pluses to release. So there are multiple sort of layers of what makes an acid strong. But you can always identify an acid if you see there is a hydrogen on the left-hand side of the chemical formula. For a base, bases are usually indicated in blue, so I'm gonna circle it in blue. They have usually written as OH minus, but sometimes HO is also. also fine. Hydroxide ions, so they end in OH and again you can have a base that contains more than one OH but these two sodium hydroxide and potassium hydroxide are probably the most common bases you'll ever encounter. These are common components of Drano or any sort of pipe cleaner reagent that you can use. ...to like flush out and clear off any sort of clogging you have in your household pipes because this stuff will just corrode and eat through whatever you have clogging up your pipes. It might also eat through your pipes. So for certain pipes, Drano is not recommended for use. Just FYI. And when you combine an acid and a base together, the components that makes it an acid, the H+, and the component that makes it a base, the OH-, will come together and make water. And because it makes neutral water, your acid bases then neutralize because the things that are acids and things that are base gets used up and they make something that is not acid and not base. So the Steps that you would go through in a titration. In this titration, we will have the base, hence the OH. In the burette, the burette is the long, tubey piece of glassware on top. We'll explain that a little bit later. And then you will have the acid down below in the flask. So that is the same picture. Here in the photo, acid is down below and then your base is in the burette. And you're going to add base into the acid until the color changes, you can see as you progress. towards the right in the series of photos or towards the right in the series of illustrations, the color of the flask changes. So when the very first hint of pink occurs in your reaction, that tells you the acid and the base have reacted and they are fully converted to water. If you keep adding more base, the... Flask will actually turn darker and darker pink and at that point you have more base than acid and you are not at neutral You are at basic. So what you're looking for is the very first instance that your flask turns pink The reason the flask turns pink is because you have added an indicator. An indicator is something that will change color when you go from acidic to basic conditions, and that color will indicate that switch. And in this case we're using an indicator called phenolphthalein. Try spelling that one. And it is clear when it's in an acidic environment, and it is pink when it is in a basic environment. So in the instance that it switches from clear to pink is when you've just gone from acid to base. So you're at your neutralization or your equivalence point. To do the calculations for this experiment, what we want is to find the molarity or the concentration of our acid. Concentration is measured as a ratio of moles two volume and that volume is in liters. So how many moles are there in a certain volume of solution? If it's more concentration or some more concentrated then there would either be less volume or more moles. You can think about this in terms of if you're making a salt water, a glass of salt water solution or sugar water. doesn't actually matter. It's just something you can taste, right? If you put a little bit of salt or sugar in your one glass of water, say, let's say it's one cup, right? Eight ounces, then it would taste a little sweet or a little salty, depending on what you use. And if you put a lot of salt or a lot of sugar in your water, then it will taste a lot sweet or a lot salty. And so that your taste buds are telling you. about how concentrated your solution is. So if you put a lot of your salt or sugar in, a lot of moles, then your molarity will get to be a larger number. And that's how you can taste a higher concentration because the flavor is stronger. You could also make the flavor stronger by adding a small amount of your salt or your sugar, but into only, say, a shot glass full of water or a spoonful of water. If you only have a little bit of volume, but you put the same number of moles, you can also see that mathematically, if you divide by a smaller number, you're going to get a higher concentration. So that's the relationship between the two. And once you have this equation in this purple box here, you can rearrange it so that you can solve for moles by using the concentration big M and the volume big L. So you can take that equation, manipulate it to solve for the thing you want. So here is the magical chemistry part. If you think about This part of the equation, once we have manipulated it, where you're taking the molarity times the volume, it will give you the moles of what you've dissolved into your solution. Maybe that's acid, maybe that's base, maybe that's salt, maybe that's sugar. But it would calculate the moles. And at the equivalence point, and this is very important, at the equivalence point, and... EQ point. Moles of acid is equal to moles of base. And since The molarity times the volume gives you moles. Then I can find the moles of acid by this side of the equation. Take the molarity concentration of the acid times the volume of the acid that I use, and that will be moles of acid. Let me highlight that. And on the other side, if I take the concentration of my base, molarity of my base, times the volume of the base, then that will give me moles of base. And these two sides are equal. So I don't actually need to know the. exact number of moles of acid or base, I just know that this relationship will hold for a acid-base neutralization reaction. So abbreviated, that equation is this guy down below. And that's probably going to be the most important equation you will use to calculate this, so I will double box it. So in your titration, you will be adding one compound to the other. This is mostly FYI, I'm not going to quiz you on these terminology, but just so you know what your instructions are telling you. So there are three parts. parts to every titration. The analyte is the solution that is in the flask. So the analyte in this reaction is the acid that's in the flask. the flask. So it's the thing you're trying to analyze, it's the thing you don't know anything about, and you're trying to find in this reaction what is the concentration of the analyte. And you want the concentration of the acid. The titrant is the thing you're adding to the analyte. The titrant is what you know. You know... that it is a base, you know its concentration, you know how much volume you're adding. This is the thing you know a lot about and you're using it to figure out something about the analyte that you don't know much about. Finally, the indicator, like I mentioned earlier, the indicator, which I should have highlighted pink, oh well, is a substance that does not react in the acid-base neutralization reaction. it doesn't react with the acid or with the base but it tells you when the endpoint is when your solution turns from acidic to basic and like I said our indicator we will use is phenolphthalein That always takes me forever to figure out exactly where all the letters go and how to spell it, so that is a good spelling challenge. Alright, so what you're going to do is do a couple of rounds of... You're going to put your acid in your flask first, and then you will look at how much of your titrant you deliver into your flask. So the volume will go from some level down to some level. And so this is your VB, the change in volume in your burette. It's going to be how much base you've added into the flask. You will know that you have added a certain amount of acid, so volume of acid. It will say, I think in your experiment, you're supposed to add 10 milliliters of acid into your flask, so you know what volume you put in. but you don't know what concentration you have. When you do the experiment, you will find the volume of base you added, because you get to choose that. And then the base that you get, the bottle will say what the concentration is. is so you also have the concentration of the base. So I will have concentration of the base, volume of the base I've added experimentally, and the volume of acid that I chose to put in the flask, and you should be able to calculate. the concentration of the acid, which was the goal of the experiment. So I just want to do some sample calculations here for you, give you a sense of what your numbers might look like. So let's... say you have added in your 10 milliliters of acid in the flask and I've color-coded it so the red is the where your acid is. acid is and purple is where your base is. For my titration, I started my burette at 0 milliliters. By the way, when you read a burette, a burette has the scale upside down. It starts at 0 at the very top and it goes to, I believe your burette goes to 50 milliliters at the bottom. With the, as you add base using your burette that level will go down and as it goes down it means you've increased the amount of base you've added so the numbers should increase. So let's say that I start at 0 milliliters and when the titration is done I am at 15.2 milliliters. So the amount has gone down to there. Just this piece of data here. And then from the bottle that you got the base from, you would have read the concentration was some number, and I've just driven that down here. So this is from bottle. Hold on. Both of these numbers are from burette. And then this number would have been from procedures. Your procedure will specifically say add this much acid. So once you have your numbers, remember the equation you're working with is M, B, V, B, M, A, V, A. And what you want to find is the molarity of your acid. So if I rearrange this to solve for what I want, I will have mA equals mB VB divided by VA. I am ready to add in my numbers. one of the beautiful things about this equation is that if you match the units for the same kind of variable so all of your volumes have the same unit and all of your concentrations have the same unit then they will cancel out. You don't have to actually convert units as long as they're all the same. So I have concentration of my base, which is on the top right corner, 1.7 molar, big M. And so if I've decided my concentration is going to be measured in molar, big M, which is the same as moles per liter, then this concentration of A is also going to be in moles. molars. And I'm going to multiply that by the volume of base I have added into the graph. If you started with your initial base volume at zero, then this is a really easy calculation. However, I just want to show you the full procedures for this, so wherever you start, you will have it, you will have the mathematical procedures. So to calculate a change in volume, how much you added, It is always going to be final minus initial. So I'm going to take my final volume, 15.2 milliliters, subtract 0 milliliters, and you can already do that in your head, it's going to be just 15.2 milliliters. So this is final minus initial. On the bottom, remember our units need to stay the same. It doesn't matter what they are, but they have to stay the same. So at the top, when I calculated volume of the base, I have picked milliliters, so that means the volume of the acid I put under the base is going to be the same. underneath will also need to be in milliliters. Good thing we already have it in milliliters. So 10.0 milliliters. So you can see mathematically the milliliters will cancel out leaving you with the unit you want. And so the concentration of the acid in this hypothetical set of data would be, give me a moment. Calculator says 2.584 and if we look at our numbers here, our sig figs should be 2. So this will turn into 2.6 molar for this experiment. You will do two titrations. You'll do a coarse titration first, and the coarse titration is just to give you a sense of what it's going to look like, about how much of your base, your NaOH, you're going to add. add in and it's pretty much a practice run right um and all of your titrations you will do are set up exactly the same way it's just a matter of how quickly you do it how carefully you do it and the first one you just want to get a sense of okay when is it going to change colors because if you're far away from the point where your color changes you do not have to do it slowly carefully you want to be approach the point where the color changes slowly so you don't overshoot if you overshoot you will have to do the titration again um the titration sorry the burette knob is a little bit touchy so if you just click on it, like a single click, it doesn't dispense in a small amount. Some of them don't dispense, so the way you make it dispense is you click and hold on the burette and then let go when you want it to stop. So give it a little time. You can also double click on the burette to get a close-up of what the reading looks like, what the scale looks like, so you can see it's upside down and you can figure out what your volume measurement should be. All right, once you're done with your coarse titration, you know roughly how much you will need to add to get to your equivalence point. You're going to be repeating the experiment, so ideally the equivalence point should occur at the same place at the same amount of volume you've dispensed. So you will do a fine titration where you get the actual exact, very carefully measured amount of volume of base so that you can do that calculation like I gave you the example earlier. So here are a little bit more detailed instructions on setting up your titration and how to be very very careful in approaching the equivalence point. You will be using a pH meter. Then the pH meter measures pH and that's this little doohickey here. And it measures pH, it measures how acidic or basic your solution is in a numerical form. And what you want is for the pH to be exactly at 7. 7 is neutral. If the pH is less than 7, it's acidic. If it's above 7, it is basic. So it's another way, in addition to your pink indicator, to give you a sense of when your titration is done. A little about pH. pH is a log-based scale. pH is calculated in using this equation right here. So if you find the concentration of the acid, then you can take the negative log of that concentration and get the pH. So in pure water, there is very, very, very little acid. This is the amount that is in pure water, 1 times 10 to the negative 7 molar, very tiny amount. If you plug it into the equation, you get pH equals 7. And you can see when the pH is low on the less than 7 end, you have things that are acidic. And when the pH is high on the more than 7 end, you have something that is basic. And And so you will use the pH meter to find when you are neither acidic or nor basic, but exactly at neutral. Alright, so your short answer questions will ask you to note down your data for your titrations, for your coarse and your fine titration. So here's the coarse, here is the fine. and you will do some calculations to show the process to find your concentration of the acid. Quick reminder that if you are asked to calculate like you are here, and remember this is the goal of your experiment, right? Please show work. I will not take off points for there not being any work if your answer is correct. But if you don't show work and your answer is incorrect, I can't help you. I can't give you partial credit for the parts that you did right, and I also can't... give you feedback about what the problem was. And it makes it really hard for me to anticipate how to help you prepare for your exam. So give me a little work. I know it's hard to type up a calculation. but show me a little work so I can see, okay, what numbers did you start with? How did you set this up? What did you divide? What did you multiply? Okay, and there are a couple of conclusion questions and then you're done. So this short answer section is shorter than usual, but don't forget you have a quiz and then also the due date for the experiment is postponed one week after what is listed on the syllabus. Thank you.

They react in a certain ratio that is specific to the compounds and that ratio is a little bit like a cooking recipe. So if you're making a grilled cheese sandwich, you know that you will probably use I would use two slices of bread and one slice of cheese. It's just most basic grilled cheese sandwich. And so the ratio of bread to cheese is two to one.

And if I want to make grilled cheese sandwiches so that I don't have any leftover bread or any leftover cheese, that would be the same procedure I would use to do a titration where I want my starting ingredients one and two to be completely used up all turned into product. Somewhat like the bread and cheese, I want all the bread and all the cheese to be used up and all made into grilled cheese sandwiches. So that's essentially the governing principle that allows us to use titrations to understand our compounds.

The goal of this experiment is to use titration to find the concentration, sometimes called the molarity, of an unknown acid or a base using a titration or volumetric techniques. And the hypothesis is that at the equivalence point where acid equals base, the amount of both are the same according to the ratio. where both is used up, the solution is now neutral.

It's neither acidic nor basic and that at that point the moles are equal and if they're equal then we can use that information to tell something about the concentration of the unknown acid or the unknown base. In this experiment you will have an unknown acid. So acid-base neutralization, we're adding an acid and a base together so that they completely react with each other and leave a neutral solution.

Some examples of things that are acids, so acids are usually indicated in red so I'm going to use red. Things that are acids have H plus ions and those are shown in the chemical formula as the H's and the front. on the left hand side of the chemical formula.

You can have more than one H, as you can see in this last one, H2SO4, this is sulfuric acid. And it contains two H plus ions, so it can release two H plus ions. One of the reasons why sulfuric acid is so corrosive and so strong, because it's got more than one H plus to release, is not the only reason.

There are weak acids that have have even more H pluses to release. So there are multiple sort of layers of what makes an acid strong. But you can always identify an acid if you see there is a hydrogen on the left-hand side of the chemical formula.

For a base, bases are usually indicated in blue, so I'm gonna circle it in blue. They have usually written as OH minus, but sometimes HO is also. also fine.

Hydroxide ions, so they end in OH and again you can have a base that contains more than one OH but these two sodium hydroxide and potassium hydroxide are probably the most common bases you'll ever encounter. These are common components of Drano or any sort of pipe cleaner reagent that you can use. ...to like flush out and clear off any sort of clogging you have in your household pipes because this stuff will just corrode and eat through whatever you have clogging up your pipes.

It might also eat through your pipes. So for certain pipes, Drano is not recommended for use. Just FYI. And when you combine an acid and a base together, the components that makes it an acid, the H+, and the component that makes it a base, the OH-, will come together and make water. And because it makes neutral water, your acid bases then neutralize because the things that are acids and things that are base gets used up and they make something that is not acid and not base.

So the Steps that you would go through in a titration. In this titration, we will have the base, hence the OH. In the burette, the burette is the long, tubey piece of glassware on top.

We'll explain that a little bit later. And then you will have the acid down below in the flask. So that is the same picture. Here in the photo, acid is down below and then your base is in the burette. And you're going to add base into the acid until the color changes, you can see as you progress.

towards the right in the series of photos or towards the right in the series of illustrations, the color of the flask changes. So when the very first hint of pink occurs in your reaction, that tells you the acid and the base have reacted and they are fully converted to water. If you keep adding more base, the... Flask will actually turn darker and darker pink and at that point you have more base than acid and you are not at neutral You are at basic. So what you're looking for is the very first instance that your flask turns pink The reason the flask turns pink is because you have added an indicator.

An indicator is something that will change color when you go from acidic to basic conditions, and that color will indicate that switch. And in this case we're using an indicator called phenolphthalein. Try spelling that one. And it is clear when it's in an acidic environment, and it is pink when it is in a basic environment. So in the instance that it switches from clear to pink is when you've just gone from acid to base.

So you're at your neutralization or your equivalence point. To do the calculations for this experiment, what we want is to find the molarity or the concentration of our acid. Concentration is measured as a ratio of moles two volume and that volume is in liters.

So how many moles are there in a certain volume of solution? If it's more concentration or some more concentrated then there would either be less volume or more moles. You can think about this in terms of if you're making a salt water, a glass of salt water solution or sugar water. doesn't actually matter.

It's just something you can taste, right? If you put a little bit of salt or sugar in your one glass of water, say, let's say it's one cup, right? Eight ounces, then it would taste a little sweet or a little salty, depending on what you use. And if you put a lot of salt or a lot of sugar in your water, then it will taste a lot sweet or a lot salty.

And so that your taste buds are telling you. about how concentrated your solution is. So if you put a lot of your salt or sugar in, a lot of moles, then your molarity will get to be a larger number. And that's how you can taste a higher concentration because the flavor is stronger. You could also make the flavor stronger by adding a small amount of your salt or your sugar, but into only, say, a shot glass full of water or a spoonful of water.

If you only have a little bit of volume, but you put the same number of moles, you can also see that mathematically, if you divide by a smaller number, you're going to get a higher concentration. So that's the relationship between the two. And once you have this equation in this purple box here, you can rearrange it so that you can solve for moles by using the concentration big M and the volume big L. So you can take that equation, manipulate it to solve for the thing you want.

So here is the magical chemistry part. If you think about This part of the equation, once we have manipulated it, where you're taking the molarity times the volume, it will give you the moles of what you've dissolved into your solution. Maybe that's acid, maybe that's base, maybe that's salt, maybe that's sugar. But it would calculate the moles.

And at the equivalence point, and this is very important, at the equivalence point, and... EQ point. Moles of acid is equal to moles of base. And since The molarity times the volume gives you moles.

Then I can find the moles of acid by this side of the equation. Take the molarity concentration of the acid times the volume of the acid that I use, and that will be moles of acid. Let me highlight that.

And on the other side, if I take the concentration of my base, molarity of my base, times the volume of the base, then that will give me moles of base. And these two sides are equal. So I don't actually need to know the.

exact number of moles of acid or base, I just know that this relationship will hold for a acid-base neutralization reaction. So abbreviated, that equation is this guy down below. And that's probably going to be the most important equation you will use to calculate this, so I will double box it. So in your titration, you will be adding one compound to the other. This is mostly FYI, I'm not going to quiz you on these terminology, but just so you know what your instructions are telling you.

So there are three parts. parts to every titration. The analyte is the solution that is in the flask. So the analyte in this reaction is the acid that's in the flask.

the flask. So it's the thing you're trying to analyze, it's the thing you don't know anything about, and you're trying to find in this reaction what is the concentration of the analyte. And you want the concentration of the acid. The titrant is the thing you're adding to the analyte.

The titrant is what you know. You know... that it is a base, you know its concentration, you know how much volume you're adding. This is the thing you know a lot about and you're using it to figure out something about the analyte that you don't know much about.

Finally, the indicator, like I mentioned earlier, the indicator, which I should have highlighted pink, oh well, is a substance that does not react in the acid-base neutralization reaction. it doesn't react with the acid or with the base but it tells you when the endpoint is when your solution turns from acidic to basic and like I said our indicator we will use is phenolphthalein That always takes me forever to figure out exactly where all the letters go and how to spell it, so that is a good spelling challenge. Alright, so what you're going to do is do a couple of rounds of...

You're going to put your acid in your flask first, and then you will look at how much of your titrant you deliver into your flask. So the volume will go from some level down to some level. And so this is your VB, the change in volume in your burette.

It's going to be how much base you've added into the flask. You will know that you have added a certain amount of acid, so volume of acid. It will say, I think in your experiment, you're supposed to add 10 milliliters of acid into your flask, so you know what volume you put in. but you don't know what concentration you have.

When you do the experiment, you will find the volume of base you added, because you get to choose that. And then the base that you get, the bottle will say what the concentration is. is so you also have the concentration of the base. So I will have concentration of the base, volume of the base I've added experimentally, and the volume of acid that I chose to put in the flask, and you should be able to calculate.

the concentration of the acid, which was the goal of the experiment. So I just want to do some sample calculations here for you, give you a sense of what your numbers might look like. So let's...

say you have added in your 10 milliliters of acid in the flask and I've color-coded it so the red is the where your acid is. acid is and purple is where your base is. For my titration, I started my burette at 0 milliliters. By the way, when you read a burette, a burette has the scale upside down.

It starts at 0 at the very top and it goes to, I believe your burette goes to 50 milliliters at the bottom. With the, as you add base using your burette that level will go down and as it goes down it means you've increased the amount of base you've added so the numbers should increase. So let's say that I start at 0 milliliters and when the titration is done I am at 15.2 milliliters. So the amount has gone down to there. Just this piece of data here.

And then from the bottle that you got the base from, you would have read the concentration was some number, and I've just driven that down here. So this is from bottle. Hold on.

Both of these numbers are from burette. And then this number would have been from procedures. Your procedure will specifically say add this much acid.

So once you have your numbers, remember the equation you're working with is M, B, V, B, M, A, V, A. And what you want to find is the molarity of your acid. So if I rearrange this to solve for what I want, I will have mA equals mB VB divided by VA. I am ready to add in my numbers.

one of the beautiful things about this equation is that if you match the units for the same kind of variable so all of your volumes have the same unit and all of your concentrations have the same unit then they will cancel out. You don't have to actually convert units as long as they're all the same. So I have concentration of my base, which is on the top right corner, 1.7 molar, big M. And so if I've decided my concentration is going to be measured in molar, big M, which is the same as moles per liter, then this concentration of A is also going to be in moles. molars.

And I'm going to multiply that by the volume of base I have added into the graph. If you started with your initial base volume at zero, then this is a really easy calculation. However, I just want to show you the full procedures for this, so wherever you start, you will have it, you will have the mathematical procedures. So to calculate a change in volume, how much you added, It is always going to be final minus initial.

So I'm going to take my final volume, 15.2 milliliters, subtract 0 milliliters, and you can already do that in your head, it's going to be just 15.2 milliliters. So this is final minus initial. On the bottom, remember our units need to stay the same. It doesn't matter what they are, but they have to stay the same. So at the top, when I calculated volume of the base, I have picked milliliters, so that means the volume of the acid I put under the base is going to be the same.

underneath will also need to be in milliliters. Good thing we already have it in milliliters. So 10.0 milliliters.

So you can see mathematically the milliliters will cancel out leaving you with the unit you want. And so the concentration of the acid in this hypothetical set of data would be, give me a moment. Calculator says 2.584 and if we look at our numbers here, our sig figs should be 2. So this will turn into 2.6 molar for this experiment.

You will do two titrations. You'll do a coarse titration first, and the coarse titration is just to give you a sense of what it's going to look like, about how much of your base, your NaOH, you're going to add. add in and it's pretty much a practice run right um and all of your titrations you will do are set up exactly the same way it's just a matter of how quickly you do it how carefully you do it and the first one you just want to get a sense of okay when is it going to change colors because if you're far away from the point where your color changes you do not have to do it slowly carefully you want to be approach the point where the color changes slowly so you don't overshoot if you overshoot you will have to do the titration again um the titration sorry the burette knob is a little bit touchy so if you just click on it, like a single click, it doesn't dispense in a small amount.

Some of them don't dispense, so the way you make it dispense is you click and hold on the burette and then let go when you want it to stop. So give it a little time. You can also double click on the burette to get a close-up of what the reading looks like, what the scale looks like, so you can see it's upside down and you can figure out what your volume measurement should be. All right, once you're done with your coarse titration, you know roughly how much you will need to add to get to your equivalence point.

You're going to be repeating the experiment, so ideally the equivalence point should occur at the same place at the same amount of volume you've dispensed. So you will do a fine titration where you get the actual exact, very carefully measured amount of volume of base so that you can do that calculation like I gave you the example earlier. So here are a little bit more detailed instructions on setting up your titration and how to be very very careful in approaching the equivalence point.

You will be using a pH meter. Then the pH meter measures pH and that's this little doohickey here. And it measures pH, it measures how acidic or basic your solution is in a numerical form.

And what you want is for the pH to be exactly at 7. 7 is neutral. If the pH is less than 7, it's acidic. If it's above 7, it is basic. So it's another way, in addition to your pink indicator, to give you a sense of when your titration is done.

A little about pH. pH is a log-based scale. pH is calculated in using this equation right here. So if you find the concentration of the acid, then you can take the negative log of that concentration and get the pH. So in pure water, there is very, very, very little acid. This is the amount that is in pure water, 1 times 10 to the negative 7 molar, very tiny amount.

If you plug it into the equation, you get pH equals 7. And you can see when the pH is low on the less than 7 end, you have things that are acidic. And when the pH is high on the more than 7 end, you have something that is basic. And And so you will use the pH meter to find when you are neither acidic or nor basic, but exactly at neutral.

Alright, so your short answer questions will ask you to note down your data for your titrations, for your coarse and your fine titration. So here's the coarse, here is the fine. and you will do some calculations to show the process to find your concentration of the acid.

Quick reminder that if you are asked to calculate like you are here, and remember this is the goal of your experiment, right? Please show work. I will not take off points for there not being any work if your answer is correct.

But if you don't show work and your answer is incorrect, I can't help you. I can't give you partial credit for the parts that you did right, and I also can't... give you feedback about what the problem was.

And it makes it really hard for me to anticipate how to help you prepare for your exam. So give me a little work. I know it's hard to type up a calculation.

but show me a little work so I can see, okay, what numbers did you start with? How did you set this up? What did you divide? What did you multiply?

Okay, and there are a couple of conclusion questions and then you're done. So this short answer section is shorter than usual, but don't forget you have a quiz and then also the due date for the experiment is postponed one week after what is listed on the syllabus. Thank you.

Transcript for:Introduction to Titrations in Chemistry

Transcript for:
Introduction to Titrations in Chemistry