A weak acid of dissociation constant 10^–5 is being titrated with aqueous NaOH solution. The pH at the point of one-third neutralization of the acid will be :

Following equilibrium exist in the weak acid solution

                             $\mathrm{HA}\rightleftharpoons {\mathrm{H}}^{+}\left(\mathrm{aq}\right)+{\mathrm{A}}^{-}\left(\mathrm{aq}\right)$

 $\mathrm{At} \mathrm{t}={\mathrm{t}}_{\frac{1}{3}}$        ${\mathrm{n}}_0-\frac{{\mathrm{n}}_{\mathrm{o}}}{3}$       $\frac{{\mathrm{n}}_{\mathrm{o}}}{3}$            $\frac{{\mathrm{n}}_{\mathrm{o}}}{3}$

After neutralization of one-third equivalents, the system is a buffer containing a weak acid and its conjugate A^-.

P^H of an acidic buffer is given as ${\text{P}}^{\mathrm{H}}={\mathrm{P}}^{{\mathrm{K}}_{\mathrm{a}}}+\log \frac{[ \mathrm{salt} ]}{\left[\mathrm{acid}\right]}=5+\log \frac{1/3}{2/3}=5-\log 2$

P^H = 4.7