TPR 6, BS Section, Q10

[puffylover]

The experimenter titrates the 48 hour old sample with 0.2M NaOH to the endpoint, using 12.4 mL of NaOH. She then back-titrates with 12.4mL 0.2M HCL and determines the pKa of the solution. What is the result of this test?

Given:
48 hour old sample has acetylsalicylic acid (pKa 4.56) and salicylic acid (pKa 2.75)

A. pKa=2.75
B. pKa=4.56
C. pKa= between 2.75 and 4.56
D. Cannot be determined from the information given

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[unique135]

D

Rationale:
1) pKa is not the property of the solution but acid in the solution. Two acids in the solution, pKa?...I doubt

2) Even if it is, the volume of solution is changing. Concentrations will change. Thus pH will change. It will never come back to original directly. I am little unclear here.



My question: can we even calculate pKa from titration?