[jackwhacky]

Hello.

I'm studying my chemistry course and this is the last lab that we have to do. The problem is that I'm studying from home and I have zero friends or teachers to help me. The lab I'm doing is based on a really bad video that the school provided and I'm doing my best to solve it.

I need to find out Ka theoretically and not from the graph.


We are performing a titration of a weak acid HAc (0,0975 mol/dm3) with NaOH. I am aware that the values in the graph are probably wrong which is due to the bad video that I MUST take my values from, so it's not my fault and I have to discuss this in my lab.

Here I calculate Ka from the graph with known pH:


Here I calculate Ka theoretically using the initial value of pH=4.08 from the graph:


My question is am I using the right method to find it out theoretically?

Obviously such a HUGE difference in values. I have no idea where to go from here. Is this even right?

Thank you so much. My future hinges on the lab. I'm ready to provide explanations if required.

[Borek]

I see nothing seriously wrong with your approach, the numbers you got are as good as the data you are basing calculations on.

The only thing you can correct is the use of 0.0974 as a concentration of HAc. Never round down intermediate results, round down only the final one.

[jackwhacky]

I see nothing seriously wrong with your approach, the numbers you got are as good as the data you are basing calculations on.

The only thing you can correct is the use of 0.0974 as a concentration of HAc. Never round down intermediate results, round down only the final one.

Thanks a lot for your reply! After I posted this I found someone with a solution that is different to mine:

Using pH at the half equivalence point


The point of the lab here is to get the pH at the starting point theoretically. What is the difference between my approach and his? Thanks 

[Meter]

I see nothing seriously wrong with your approach, the numbers you got are as good as the data you are basing calculations on.

The only thing you can correct is the use of 0.0974 as a concentration of HAc. Never round down intermediate results, round down only the final one.

Thanks a lot for your reply! After I posted this I found someone with a solution that is different to mine:

Using pH at the half equivalence point


The point of the lab here is to get the pH at the starting point theoretically. What is the difference between my approach and his? Thanks 

If you know the pKa of any acid, you can simply calculate Ka using Ka = 10^-pKa. The problem is, you're not allowed to this by using your graph?

Then I think your second approach in your original post is good. Using pH = 4.08 as an initial value works because it's ideally a good measurement. The reason you get a significant deviation is probably because the data is poor. Also, if you want a slightly more accurate result, you should subtract x in the denominator of your equation, as this is the amount of the weak acid that is converted. And don't round intermediate values as Borek pointed out Also, Ka doesn't have a unit by convention, but do what your teacher wants you to do.

[Borek]

x in the second approach has nothing to do with Ka, it is more like an approximate pH calculated using Ka value estimated from the titration curve, doesn't make much sense.

[Meter]

x in the second approach has nothing to do with Ka, it is more like an approximate pH calculated using Ka value estimated from the titration curve, doesn't make much sense.

What I meant is that he should subtract 8.317... e-5 from the denominator when calculating Ka in his second approach as this is what you generally do with x in equivalent problems where Ka is known and pH is not. But considering the difference in order of magnitude, it probably makes a marginal difference anyway.

Edit: Never mind all that, it seems like he implicitly did exactly that. My bad.

[jackwhacky]

I see nothing seriously wrong with your approach, the numbers you got are as good as the data you are basing calculations on.

The only thing you can correct is the use of 0.0974 as a concentration of HAc. Never round down intermediate results, round down only the final one.

Thanks a lot for your reply! After I posted this I found someone with a solution that is different to mine:

Using pH at the half equivalence point


The point of the lab here is to get the pH at the starting point theoretically. What is the difference between my approach and his? Thanks 

If you know the pKa of any acid, you can simply calculate Ka using Ka = 10^-pKa. The problem is, you're not allowed to this by using your graph?

Then I think your second approach in your original post is good. Using pH = 4.08 as an initial value works because it's ideally a good measurement. The reason you get a significant deviation is probably because the data is poor. Also, if you want a slightly more accurate result, you should subtract x in the denominator of your equation, as this is the amount of the weak acid that is converted. And don't round intermediate values as Borek pointed out Also, Ka doesn't have a unit by convention, but do what your teacher wants you to do.

x in the second approach has nothing to do with Ka, it is more like an approximate pH calculated using Ka value estimated from the titration curve, doesn't make much sense.

Thank you so much for taking your time, you guys are life savers. Can't thank you enough.

Yeah it's true, I implicitly subtracted the denominator. The second approach where I get the value of x really didn't make sense for me either, but I just wanted to make sure that my original thought was clearer. Thanks again!