So the goal of this experiment is to determine the concentration of a sodium hydroxide solution through a titration with solid oxalic acid. Okay, so what we're going to do is we're going to take some oxalic acid and I'll draw out the structure of this. Okay, so from...
the lecture material, we would look at this structure and we would decide that it has two acidic hydrogens. So both of these hydrogens are going to come off and they're going to neutralize sodium hydroxide. Alright, so the reaction that we're looking at is this. So we're going to take oxalic acid and the way that we would write this is we would put those two acidic hydrogens out in front. Okay, and we're going to dissolve this in some water.
So it's going to become aqueous. And then we're going to add this to sodium hydroxide, which will be aqueous. We're trying to determine the concentration of the sodium hydroxide solution.
Now in the lecture, I mentioned that kind of the way that I do this is I look at how many hydrons we have, which is two. And then what I do is I look at how many OH-s I need. And in this case, I'm going to need two of those.
I need one OH for each H. So I'm going to need two of these. And then that's going to produce two water molecules.
And then what we're going to be left with is Sodium oxalate, which will be aqueous. So this is the chemical reaction that we're going to be interested in. And what we are trying to determine is we are trying to determine the concentration of sodium hydroxide.
Right, so we don't know what this is. Now in order to do this, we're going to need to know the moles of sodium hydroxide, and we're going to divide this by a volume of sodium hydroxide. Okay, so I'm going to put here an asterisk next to the moles of sodium hydroxide.
And notice that I've made that in purple because it turns out that the way we're going to figure out how many moles of sodium hydroxide, this is going to be determined by the amount of oxalic acid we put in. Okay, and so the idea behind this is that we're going to have a certain mass of oxalic acid. And if we know the mass of this, we can figure out the moles of oxalic acid.
And then once we know the moles of oxalic acid, if we use the balanced equation, we know that we're going to need twice as many moles of sodium hydroxide. So the moles of sodium hydroxide are going to be determined by the mass of the oxalic acid. Okay. And then the volume of sodium hydroxide. This is going to be what is dispensed from the burette.
Okay, and that'll make more sense in the next slide. So in order to figure out the concentration of sodium hydroxide, we need two things. We need the moles per liter. And so again, we're going to determine the moles based on the mass of oxalic acid, and we're going to determine the volume based on how much of this sodium hydroxide comes out of the burette.
Okay, now again, a lot of the math on how to do this and the theory behind it comes from our lecture, so I'm going to rely on you guys knowing and understanding the lecture. knowing how to do the homework. Okay, so this is the plan.
So what we're going to do is we're going to take our sodium hydroxide solution and we're going to put it into what's called a burette. And we don't know what the concentration of this sodium hydroxide is. Then in an Erlenmeyer flask, we're going to some amount, we'll call it X right now, some amount of grams of oxalic acid and we're going to dissolve it in 25 milliliters of water. And this 25 milliliters here, by the way, this number It doesn't mean anything to us. We're not going to use that in a calculation, so please don't try to force that into anything.
This is just us trying to dissolve the oxalic acid so that it's readily available to react with the sodium hydroxide. This is what matters. What is the mass of oxalic acid?
Okay, it doesn't matter. We could dissolve it in 30 milliliters of water and that will not affect how this experiment is run. Okay, and then notice that the way that I've kind of drawn this is that this is kind of a clear solution right now.
Okay, there's no color to it. And oxalic acid is a clear colorless solution. Sodium hydroxide is a clear and colorless solution. So when we mix these two together, we're going to add something. called phenolphthalein, which is going to turn the solution pink when the chemical reaction is balanced, meaning you have one mole of oxalic acid for every two moles of sodium hydroxide.
So we're going to do three runs. And basically what I'm going to do is I have some pictures drawn that are going to give you kind of the results. And then you're going to end up interpreting these in a series of questions that will be posted to go along with this assignment. Okay, so... for trial number one.
So notice, here we go, we have our burette, and I have like kind of a blown up version of the initial volume of the sodium hydroxide. So remember the sodium hydroxide is in the burette. And then we open up the stopcock, which is this piece right here, and we let in the sodium hydroxide, and we let it mix with the oxalic acid in the Erlenmeyer flask, and notice that it turned pink and so I stopped that and when I was all done this is a reading of the final volume okay now notice this is important when you do your calculations notice that the readings on the burette the numbers increase as you go down okay so for instance this is one that I will read for you but if I was going to read That number, that would be read as 3.16, perhaps. Okay. A common mistake is people will read it as maybe 4.85 because they're used to reading other pieces of equipment.
Okay. Now, in this trial, When I put in the oxalic acid into the flask, I put 0.2911 grams of oxalic acid. went into this flask.
And this is specific for this trial. So I put 0.2911 grams of oxalic acid into there. I added my 25 milliliters of water and I dissolved it.
And then I did the reaction where I added sodium hydroxide. From this information and understanding how to do the calculations, we should be able to calculate the concentration of the sodium hydroxide for trial number one. And this is something that you're going to be asked to input onto. a canvas assignment. Okay, next, trial number two.
Exact same setup. The numbers are a little bit different here, but it's the exact same setup. And in this example, I measured out 0.3312 grams of oxalic acid.
and I placed them into this flask dissolved in water, did the titration until it turned light pink, and then again from this information, initial volume, final volume of sodium hydroxide, I should be able to calculate my concentration for trial number two of the sodium hydroxide. And then for trial three, same exact setup again, but in this case I used that much oxalic acid, 0.2874 grams. I have my initial volume of sodium hydroxide, I have my final volume of sodium hydroxide, and again I should be able to, using this information, calculate the concentration of sodium hydroxide for trial three. So this is pretty standard where we would do three runs and then we will end up taking our sodium hydroxide concentration from all three runs and then we will average that.
And then what we're going to do in the next lab is we're going to take. our average sodium hydroxide concentration, we're going to use that to determine an acid concentration that we don't know. Okay.
So this kind of ends all the stuff I have to talk about for this first part of this acid base lab that we're doing. What I would do is, so you're going to be going in and you're going to be doing this exercise for... the in canvas to try to figure out the concentration of sodium hydroxide. So you're going to need to know these numbers.
So you may need to pause and write them down, like rewatch this, look at how much acid was added, what was the volume of the sodium hydroxide, and so on. And then you're going to use those numbers in calculations that you're going to do on the canvas assignment. Okay.
All right.