A Buffer Was Prepared Such That the PH Was 5

A buffer was prepared such that the pH was 5.20. If the solution contains 0.400 moles of acetic acid (CH₃COOH) per liter, what is the concentration of the acetate ion (CH₃COO ^- ) in this solution?
The p K _a for acetic acid is 4.74.

A) 0.40 M
B) 0.14 M
C) 4.2×10 ^{- 3} M
D) 6.3×10 ^{- 6} M
E) 1.1 M