Determine Ka1 and Ka2 of Diprotic Acid: Titration Problem

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"A student was given a .1 M solution of an unknown diprotic acid, H2A, and asked to determine the Ka1 and Ka2 values. The student titrated 50.0 mL of the .1 M H2A with .1 M NaOH. After 25mL of NaOH was added, the pH of the resulting solution was 6.7. After 50 mL of NaOH was added, the pH of the resulting solution was 8.00. What are the kalues of Ka1 and Ka2?"

I'm a bit lost with this problem. Are the volumes that are given mean it has reached the stoichiometric point? If so, I think I can figure that out just through equilibrium expressions, but I'm not entirely sure. Any help would be appreciated. Thanks!
 
  • #2
Good question. What does the titration curve look like? What are the other points which aid in graphing the curve? I believe the problem description author wants you to use pH 6.7 for the first end point and pH 8 for the second end point.

Also, this may seem too picky; but the author probably means 0.1 FORMAL diprotic acid (according to how the solution was prepared. The actual MOLARITY may be different due to hydrolysis).
 
  • #3
Check stoichiometry of the reaction. Do you know what Henderson-Hasselbalch equation is? Do you know when pH of the solution equals pKa?
 

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