Titration

[Guest oxaloacetate]

I have started reviewing for the August MCAT and I have come across a question that has stumped me. The question reads "Based on the data in the table and the titration curve shown in the graph, which of the following is the unknown acid?

 

The passage starts with: The following graph shows a titration of 100mL of an unknown weak acid of undetermined concentration with 0.20 M NaOH.

 

The graph is a typical titration of a weak acid. Starting pH = 2, End point/Equivalence point at approximately 50 mL NaOH.

 

The table supplies the following information:

Hydrogen Sulfate Ka 1.2 x 10 -2 pKa 1.92

Formic Ka 1.8 x 10 -4 pKa 3.74

Acetic Ka 1.8 x 10 -5 pKa 4.74

Dihydrogen Phosphate 6.2 x 10 -8 pKa 7.21

 

Based on this information I was able to conclude that the acid was not dihydrogen phosphate (since only one pKa appeared in the titration graph) or hydrogen sulphate (since it is the strongest acid given). However, I need help figuring out how to detemine if the acid is formic acid or acetic acid and I am not sure how to approach this....Does anyone have any suggestions?!?!:rolleyes

[Guest Biochem10]

Hi.

 

First find the pH at which the equivalence point occurs and the volume of NaOH added at that point. (stated in the problem 50 mL) The pH = pKa when NaOH is half of that at the equivalence point. So, look for the pH when NaOH is at 25 ml, this should be equal to the pKa of the acid being titrated.

 

Hope this helps!

[Guest oxaloacetate]

Thanks Biochem...I got the right answer using the information you provided me! It is crazy how much you can forget after being out of university for a few years! Thanks again...