Phosphoric Acid, H₃PO₄, Is a Triprotic Acid, for Which K_a1

Multiple Choice

Phosphoric acid, H₃PO₄, is a triprotic acid, for which K_a1 = 7.2 * 10¯³, K_a2 = 6.3 * 10¯⁸ and K_a3 = 4.2 * 10¯¹³. What is the value of K_b for the hydrogen phosphate anion, HPO₄²¯?

A) 6.3 * 10¯⁸
B) 4.2 * 10¯¹³
C) 1.4 * 10¯¹²
D) 1.6 * 10¯⁷
E) 2.4 * 10¯²