What Is the Hydronium Ion Concentration in a Solution Prepared $0.030 \mathrm {~mol} \mathrm {~L} ^ { - 1 }$

Multiple Choice

What is the hydronium ion concentration in a solution prepared by mixing 50.00 mL of 0.10 mol L^-1 HCN with 50.00 mL of $0.030 \mathrm {~mol} \mathrm {~L} ^ { - 1 }$ NaCN? Assume that the volumes of the solutions are additive and that K_a = $4.9 \times 10 ^{- 10} \text { for } \mathrm { HCN } \text {. }$

A) 1.5 × 10^-10 mol L^-1
B) 4.9 × 10^-10 mol L^-1
C) 1.6 × 10^-9 mol L^-1
D) 7.0 × 10^-6 mol L^-1