Question

A 350 mL saturated solution of magnesium hydroxide, Mg(OH)2 is prepared at 25 degrees Celsius. The solubility product constant, Ksp, for Mg(OH)2 is 1.8 x 10^-11 at 25 degrees Celsius.
1) Find the molar solubility of Mg(OH)2 at 25 degrees Celsius?

2) What is the concentration (mol/L) of hydroxide ion, OH-, in the saturated solution at 25 degrees Celsius?

Answer 1

Answer:

For 1: The solubility of magnesium hydroxide at 25°C is $1.65\times 10^{-4}M$

For 2: The concentration of hydroxide ion in the saturated solution at 25°C is $3.3\times 10^{-4}M$

Explanation:

• For 1:

The equation for the ionization of the magnesium hydroxide is given as:

             $Mg(OH)_2\leftrightharpoons Mg^{2+}+2OH^-$

                                    s            2s

Expression for the solubility product of $Mg(OH)_2$ will be:

$K_{sp}=[Mg^{2+}][OH^-]^2\\\\K_{sp}=s\times (2s)^2=4s^3$

We are given:

$K_{sp}=1.8\times 10^{-11}$

Putting values in above equation, we get:

$1.8\times 10^{-11}=4s^3\\\\s=1.65\times 10^{-4}M$

Hence, the solubility of magnesium hydroxide at 25°C is $1.65\times 10^{-4}M$

• For 2:

Concentration of hydroxide ion in the solution = 2s = $(2\times 1.65\times 10^{-4})=3.3\times 10^{-4}M$

Hence, the concentration of hydroxide ion in the saturated solution at 25°C is $3.3\times 10^{-4}M$