Question

pH of aqueous ${{\rm{K}}_{\rm{2}}}{\rm{S}}$ solution is:
A) >7
B) 7
C) <7
D) 14

Answer 1

Hint: We know that, ${{\rm{K}}_{\rm{2}}}{\rm{S}}$ is the salt formed from the reaction of KOH (strong electrolyte) and hydrogen sulphide. Water is also formed in the reaction. The reaction is,
${\rm{KOH}} + {{\rm{H}}_{\rm{2}}}{\rm{S}} \to {{\rm{K}}_{\rm{2}}}{\rm{S}} + {{\rm{H}}_2}{\rm{O}}$.

Complete step by step answer:
Here, we have to calculate the pH of aqueous ${{\rm{K}}_{\rm{2}}}{\rm{S}}$ solution. ${{\rm{K}}_{\rm{2}}}{\rm{S}}$ indicates the presence of sulphide ions $\left( {{{\rm{S}}^{2 - }}} \right)$. On reaction of sulphide ions and water molecules, formation of hydrogen sulphide and hydroxide ions takes place.
${{\rm{S}}^{2 - }} + {{\rm{H}}_{\rm{2}}}{\rm{O}} \to {{\rm{H}}_2}{\rm{S}} + {\rm{O}}{{\rm{H}}^ - }$
The formation of hydroxide ions in the solution indicates the presence of base. So, the value of pH of aqueous solution of ${{\rm{K}}_{\rm{2}}}{\rm{S}}$ is greater than 7.
Hence, the correct answer is option A.

Additional Information:
Let’s discuss pH in detail. pH is the negative logarithm (base 10) of its hydronium ion concentration. The mathematical expression of pH is,
${\rm{pH}} = - {\log _{10}}\left[ {{{\rm{H}}_{\rm{3}}}{{\rm{O}}^ + }} \right]$
We can also the above expression as
${\rm{pH}} = \dfrac{1}{{{{\log }_{10}}\left[ {{{\rm{H}}_{\rm{3}}}{{\rm{O}}^ + }} \right]}}$
So, another definition of pH is that it is the logarithm of the reciprocal hydrogen ion concentration.
pH scale is the range for expressing the nature of any solution. At 298 K, concentration hydronium ion $\left( {{{\rm{H}}_{\rm{3}}}{{\rm{O}}^ + }} \right)$ ranges from ${10^0}\,{\rm{M}}$ to ${10^{ - 14}}\,{\rm{M}}$. The corresponding pH range is from 0 to 14.
For a neutral solution, $\left[ {{{\rm{H}}_{\rm{3}}}{{\rm{O}}^ + }} \right]$=${10^{ - 7}}$ and value of pH is equal to 7
For an acidic solution, $\left[ {{{\rm{H}}_{\rm{3}}}{{\rm{O}}^ + }} \right]$>${10^{ - 7}}$ and value of pH is less than 7
For a basic solution, $\left[ {{{\rm{H}}_{\rm{3}}}{{\rm{O}}^ + }} \right]$<${10^{ - 7}}$ and value of pH is greater than 7

Note:
The nature of the solution can also be expressed in terms of pOH value. For a neutral solution value of pOH is equal to 7, for an acidic solution it is more than 7 and for a basic solution value of pOH is less than 7.