calculate the pH for this buffer.

Question

A buffer consists of 0.13 M Na₂HPO₄ and 0.31 M Na₃PO₄. Given that the K values for H₃PO₄ are, K_a1 = 7.2 x 10^-3, K_a2 = 6.3 x 10^-8, and K_a3 = 4.2 x 10^-13, calculate the pH for this buffer.

Answer 1

Henderson Hasselbalch equation...

pH = pKa + log [salt]/[acid]

In this problem, the acid is HPO₄^2- (Na₂HPO₄)

The salt is PO₄^3- (Na₃PO₄)

The Ka = 4.2x10^-13 and pKa = 12.4

pH = 12.4 + log (0.31/0.13)

pH = 12.4 + 0.38

pH = 12.8