The Acid- Dissociation Constants of Phosphoric Acid (H₃PO₄) Are K_a1

The acid- dissociation constants of phosphoric acid (H₃PO₄) are K_a1 = 7.5 × 10^{- 3}, K_a2 = 6.2 × 10^{- 8}, and K_a3 = 4.2 × 10^{- 13} at 25.0 °C. What is the pH of a 2.5 M aqueous solution of phosphoric acid?

A) 0.13
B) 2.5
C) 0.40
D) 1.8
E) 0.87