Question

At 25º C, the solubility product of $Mg{{(OH)}_{2}}$ is $1.0\times {{10}^{-11}}$, At which pH, will $M{{g}^{2+}}$ ions start precipitating in the form of $Mg{{(OH)}_{2}}$ from a solution of 0.001 M $M{{g}^{2+}}$ ions?
A. 9
B. 10
C. 11
D. 8

Answer 1

Hint: The solubility product is one of a type of equilibrium constant whose value depends on temperature. It is denoted by ${{K}_{sp}}$. Solubility products usually increase with an increase in temperature due to increased solubility.

Complete Step by step solution: Solubility is defined as that property of a substance in which solute is dissolved in a solvent in order to form a solution. Solute is that substance which gets dissolved (present in small amount) and solvent in which solute gets dissolved (present in higher amount). The solubility product constant is the equilibrium constant for the dissolution of a solid substance into an aqueous solution. Solubility depends on a number of parameters out of which lattice enthalpy of salt and solvation energy of ions in the solution are most important.
Solubility product of $Mg{{(OH)}_{2}}$can be calculated by:
${{({{K}_{sp}})}_{Mg{{(OH)}_{2}}}}={{[Mg]}^{2+}}{{[O{{H}^{-}}]}^{2}}$
Solubility product of $Mg{{(OH)}_{2}}$is $1.0\times {{10}^{-11}}$ and concentration of $M{{g}^{2+}}$is 0.001 M given, put these values in the equation:
$1\times {{10}^{-11}}=[0.001]{{[O{{H}^{-}}]}^{2}}$
${{[O{{H}^{-}}]}^{2}}=\dfrac{{{10}^{-11}}}{{{10}^{-3}}}={{10}^{-8}}$
$[O{{H}^{-}}]={{10}^{-4}}$
$pOH=4$
Thus, $pH=14-4=10$
At pH 10 $M{{g}^{2+}}$ ions will start precipitating in the form of $Mg{{(OH)}_{2}}$from a solution of 0.001 M $M{{g}^{2+}}$ions.

Option B is the correct answer.

Note: pH and pOH are generally a quantity which is used to measure how acidic or how basic a solution is and for most solutions it is measured on a scale of 0 to 14. The lower the number on the pH scale the more acidic the solution is and higher the number on pH scale, more basic is the solution. Neutral solutions which are not acidic or basic have a pH of 7. pH is measured on a logarithmic scale.