Answer
$[OH^-] \approx 3.1623 \times 10^{-3}$
Work Step by Step
1. Find the pOH of the solution:
$pH + pOH = 14$
$11.5 + pOH = 14$
$ pOH = 14 - 11.5$
$ pOH = 2.5$
2. Calculate the concentration of $OH^-$.
$[OH^-] = 10^{-pOH}$
$[OH^-] = 10^{-2.5}$
$[OH^-] \approx 0.0031623$
