Answer
a.$[OH^-] = 2.5 \times 10^{-13} \space M$
b.$[OH^-] = 2.0 \times 10^{-9} \space M$
c. $[OH^-] = 5.0 \times 10^{-11} \space M$
d.$[OH^-] = 1.3 \times 10^{-6} \space M$
Work Step by Step
a.$$[OH^-] = \frac{1.0 \times 10^{-14}}{[H_3O^+]} = \frac{1.0 \times 10^{-14}}{ 4.0 \times 10^{-2} } = 2.5 \times 10^{-13} \space M$$
b.$$[OH^-] = \frac{1.0 \times 10^{-14}}{[H_3O^+]} = \frac{1.0 \times 10^{-14}}{ 5.0 \times 10^{-6} } = 2.0 \times 10^{-9} \space M$$
c. $$[OH^-] = \frac{1.0 \times 10^{-14}}{[H_3O^+]} = \frac{1.0 \times 10^{-14}}{ 2.0 \times 10^{-4} } = 5.0 \times 10^{-11} \space M$$
d.$$[OH^-] = \frac{1.0 \times 10^{-14}}{[H_3O^+]} = \frac{1.0 \times 10^{-14}}{ 7.9 \times 10^{-9} } = 1.3 \times 10^{-6} \space M$$
