Answer
$ [Sr(OH)_2] = 1.581\times 10^{- 4}M$
Work Step by Step
1. Calculate $[OH^-]$:
pH + pOH = 14
10.5 + pOH = 14
pOH = 3.5
$[OH^-] = 10^{-pOH}$
$[OH^-] = 10^{- 3.5}$
$[OH^-] = 3.162 \times 10^{- 4}$
2. Since $Sr(OH)_2$ is a strong base with 2 OH in each molecule:
$[OH^-] = 2 * [Sr(OH)_2]$
$ 3.162\times 10^{- 4} = 2 * [Sr(OH)_2]$
$ \frac{ 3.162\times 10^{- 4}}{ 2} = [Sr(OH)_2]$
$ 1.581\times 10^{- 4}M = [Sr(OH)_2]$
