If acetic acid mixed with sodium acetate, then ${\text{H}}^{+}$  ion concentration will

 Due to common ion effect the concentration of ${\text{H}}^{+}$  ion is decreased.

          $\begin{array}{c}{\text{CH}}_{\text{3}}\text{COOH}\text{(}l\text{)}\rightleftharpoons {\text{CH}}_{\text{3}}{\text{COO}}^{-}\left(aq\right)+{\text{H}}^{+}\left(aq\right)\\ {\text{CH}}_{\text{3}}\text{COONa}\left(s\right)\rightleftharpoons {\text{CH}}_{\text{3}}{\text{COO}}^{-}\left(aq\right)+{\text{Na}}^{+}\left(aq\right)\end{array}$

If acetic acid is mixed with sodium acetate then ${\text{H}}^{+}$  ion is consumed in a reaction to form undissociated weak acid. Therefore, the concentration of ${\text{H}}^{+}$  ion is decreased.