Question

In the dissociation of $N{H}_{4}OH$,
$N{H}_{4}OH\left(aq\right)\rightleftharpoons N{H}_4^{+}\left(aq\right)+O{H}^{-}\left(aq\right)$
If $N{H}_{4}Cl$ is added to it, how will the $pH$ for the $N{H}_{4}OH$ solution change?

• A. pH remains the same.
• B. pH increases.
• C. pH decreases.
• D. pH may increase or decrease depending on the concentration of $N{H}_{4}Cl$ added.

Answer 1

The correct option is C pH decreases.

The dissociation reaction for the weak base , $N{H}_{4}OH$ is :$N{H}_{4}OH\left(aq\right)\rightleftharpoons N{H}_4^{+}\left(aq\right)+O{H}^{-}\left(aq\right)$
Now, Ammonium chloride is added to the solution. It is a strong electrolyte, which ionizes almost completely as follows: $N{H}_{4}Cl\left(aq\right)⟶N{H}_4^{+}\left(aq\right)+C{l}^{-}\left(aq\right)$
So with addition of $N{H}_{4}Cl$ the concentration of $N{H}_4^{+}$ is increased. Due to the common ion effect, the equilibrium shifts towards backward direction i.e the dissociation of $N{H}_{4}OH$ is reduced .
Consequently the concentration of $O{H}^{-}$ ions is decreased. As a result the basic character of the medium decreases or acid character increases.
If acid character increases the $pH$ of the medium will decrease.