Arsenic acid is a tripotic acid. What is the pH of a 0.035 M solution of arsenic acid?

Determine the pH of 0.75 M Na₃PO₄ if the K_a values are given as:

K_a1 = 5.7 × 10^{− 3}, K_a2 = 1.1 × 10^{− 7}, K_a3 = 3.2 × 10^{− 12}