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

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

${\mathrm{CH}}_3{\mathrm{COOH}}_{\left(\mathrm{l}\right)}\to {\mathrm{CH}}_3{{\mathrm{COO}}^{-}}_{\left(\mathrm{aq}\right)}+{{\mathrm{H}}^{+}}_{\left(\mathrm{aq}\right)}$

${\mathrm{CH}}_3{\mathrm{COOH}}_{\left(\mathrm{s}\right)}\to {\mathrm{CH}}_3{{\mathrm{COO}}^{-}}_{\left(\mathrm{aq}\right)}+{{\mathrm{Na}}^{+}}_{\left(\mathrm{aq}\right)}$If acetic acid is mixed with sodium acetate then ${\mathrm{H}}^{+}$ ion is consumed in a reaction to  form undissociated weak acid. Therefore, the concentration of ${\mathrm{H}}^{+}$ion is decreased.