What Is the Hydroxide Ion Concentration in a Solution of 0.40

Multiple Choice

What is the hydroxide ion concentration in a solution of 0.40 M hypochlorous acid (HOCl) ?
K _a = 3.0×10 ^{- 8}

A) 1.3×10 ^{- 8}
B) 1.1×10 ^{- 4}
C) 9.1×10 ^{- 11}
D) 2.5×10 ^{- 14}
E) none of these