I am having a hard time answering part C of this question. A) When titrated with

Question

I am having a hard time answering part C of this question.

A) When titrated with a 0.1155 M   solution of sodium hydroxide, a 50.00 mL solution of an unknown polyprotic acid required 23.92 mL   to reach the first equivalence point. Calculate the molar concentration of the unknown acid.

ANSWER: 5.526×102 M

B) The titration curve was found to have three equivalence points. What volume of the sodium hydroxide solution was necessary to fully titrate the unknown acid solution?

ANSWER: 71.76 mL

C) The following data was collected from the titration curve. Identify the values for pKa1, pKa2, and pKa3.

Volume (mL) pH        0 1.95 11.96   2.45   23.92   5.27   35.88   7.42   47.84   8.39   59.80   9.75   71.76   10.54

Explanation / Answer

Titration

C) pH at 1/2 equivalence point = pKa

So,

first 1/2 equivalence point will be reached at NaOH volume = 11.96 ml

pH = 2.45

pKa1 = 2.45

second 1/2 equivalence point will be reached at NaOH volume = 35.88 ml

pH = 7.42

pKa2 = 7.42

Third 1/2 equivalence point will be reached at NaOH volume = 59.80 ml

pH = 9.75

pKa3 = 9.75

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