If an Aqueous Solution Is Prepared by Dissolving 1

If an aqueous solution is prepared by dissolving 1.6 mol Na₂CrO₄ in 1.0 L of a buffer at pH 8.24,what will be [Cr₂O₇^2-] in the solution? The resultant equilibria are:
HCrO₄^-(aq) ⇌ H⁺(aq) + CrO₄^2-(aq) K_a = 3.2 × 10^-7
2 HCrO₄^-(aq) ⇌ Cr₂O₇^2-(aq) + H₂O(l) K = 33

A) 9.9 × 10^-5 M
B) 2.9 × 10^-28 M
C) 8.9 × 10^-9 M
D) 8.6 × 10² M
E) 2.8 × 10^-2 M