Question

The solubility of $Ca{\left(OH\right)}_2$ is $1.50kg$ $m^{-3}$ at ${25}^{o}C$. Assume $100\%$ dissociation. What is the $pH$ of a saturated solution of $Ca{\left(OH\right)}_2$?

• A. $12.6$
• B. $7$
• C. $8$
• D. $10$

Answer 1

The correct option is A $12.6$

The solubility $S$ of calcium hydroxide is $1.50$ $kg{m}^{-3}$ or $1.50$ $g{L}^{-1}$.

Converting the unit of solubility in moles per litre:

$\frac{1.50g{L}^{-1}}{74.1gmo{l}^{-1}}=0.020mol{L}^{-1}$.

Th hydroxide ion concentration is twice its solubility. Therefore,

$\left[O{H}^{-}\right]-=2S=2\times 0.020M=0.040M$

The pOH of the solution is $pOH=-log\left[O{H}^{-}\right]=-log0.040=1.393$.

The pH of the solution is $pH=14-pOH=14-1.393=12.607$.

Hence, the pH of the solution is $12.607$.