Question

At a concentration of 1 M, the weak acid HNO2 is 2% ionized and the pH of the solution is 1.7. What happens when KNO2(s) is dissolved into the solution?

Answer 1

Concepts and reason

This problem is based on the concept of pH and Le Chatelier's principle which states that “when the reaction is already there at equilibrium and it is subjected to a change of concentration, pressure or temperature, the equilibrium shifts in the direction that tends to undo the effect of the change”.

Fundamentals

Le Chatelier's principle states that “when the reaction is already there at equilibrium and it is subjected to a change of concentration, pressure or temperature, the equilibrium shifts in the direction that tends to undo the effect of the change”

pH is defined as the measure of the hydrogen ion concentration of the solution.

It is calculated as follows:

${\rm{pH}} = - \log \left[ {{{\rm{H}}^ + }} \right]$

Ionization of weak acid is as follows:

On adding ${\rm{KN}}{{\rm{O}}_2}$, it will furnish two ions ${{\rm{K}}^ + }$ and ${\rm{NO}}_2^ -$. Therefore, when ${\rm{KN}}{{\rm{O}}_2}$ is dissolved in the solution the ${\rm{NO}}_2^ -$ concentration increases.

So, when ${\rm{KN}}{{\rm{O}}_2}$ is added, equilibrium shifts to left according to Le Chatelier's principle.

As the equilibrium shifts towards left, the extent of ${\rm{HN}}{{\rm{O}}_2}$ ionization decreases.

Since the equilibrium shifts towards left, the extent of ${\rm{HN}}{{\rm{O}}_2}$ ionization decreases. This leads to a decrease in concentration of ${{\rm{H}}^ + }$.

Since, ${\rm{pH}} = - \log \left[ {{{\rm{H}}^ + }} \right]$, when ${{\rm{H}}^ + }$ concentration decreases, the pH value increases.

Ans:

The extent of ${\rm{HN}}{{\rm{O}}_2}$ ionization decreases.

The concentration of ${{\rm{H}}^ + }$ decreases.

The pH value increases.