For H₃PO₄, a Tripotic Acid, K_a1 = 7

For H₃PO₄, a tripotic acid, K_a1 = 7.1 × 10^-3, K_a2 = 6.3 × 10^-8, and K_a3 = 4.5 × 10^-13. What is the pH of a solution that contains 0.200 moles of K₂HPO₄ and 0.300 moles of Na₃PO₄ per liter of solution?Hint: Treat this as a buffer and use the K_a that matches the given solutions.

A) 7.20
B) 7.34
C) 12.17
D) 12.35
E) 12.52