Question

Calculating Ka from titration curve.

We titrated H₃PO₄ with NaOH. We began with 40.0 mL 0.0970 MH₃PO_4, titrated with 0.2085 M NaOH. We reached first equivalence at 18.78 mL NaOH titrated, and second equivalence at 38.50 mL titrated.

Calculate Ka₁ for H₃PO₄ using the following data obtained from a titration curve:

1) From the initial pH = 1.97
2) From the pH value half way to the first equivalence point = 2.10

Calculate Ka₂ for H₃PO₄ from the following data obtained from a titration curve:

1) From the pH at the first equivalence point = 4.10
2) From the pH half way to the second equivalence point = 7.00

Answer 1

From the pH value half way to the first equivalence point = 2.10

H3PO4 is triprotic acid. so it has 3 Ka values.

Ka1 , Ka2 , Ka3 .

At half way to the first equivalence point :

pH = pKa1

pH = 2.10

pKa1 = 2.10