(આપેલ છે: $\mathrm{E}_{\mathrm{Sn}^{2+} / \mathrm{Sn}}^{0}=-0.14 \mathrm{\;V}$ $\left.\mathrm{E}_{\mathrm{Pb}^{+2}/{\mathrm{Pb}}}^{0}=-0.13 \;\mathrm{V}, \frac{2.303 \mathrm{RT}}{\mathrm{F}}=0.06\right)$
$\mathrm{Sn}(\mathrm{s})+\mathrm{Pb}^{+2}(\mathrm{aq}) \rightarrow \mathrm{Sn}^{+2}(\mathrm{aq})+\mathrm{Pb}(\mathrm{s})\dots (1)$
Apply Nernst equation:
$\mathrm{E}_{\text {cell}}=\mathrm{E}_{\text {cel }}^{0}-\frac{0.06}{2} \log \frac{\left[\mathrm{Sn}^{+2}\right]}{\left[\mathrm{Pb}^{+2}\right]}$
$\mathrm{E}_{\mathrm{cell}}^{0}=-0.13+0.14=0.01 \mathrm{V}$
At equilibrium : $E_{\text {cell}}=0$
Substituting in $( 1)$
$0=0.01-\frac{0.06}{2} \log \frac{\left[\mathrm{Sn}^{+2}\right]}{\left[\mathrm{Pb}^{+2}\right]}$
$\Rightarrow \quad \frac{1}{3}=\log \frac{\left[\mathrm{Sn}^{+2}\right]}{\left[\mathrm{Pb}^{+2}\right]}$
$\Rightarrow\; \frac{\left[\mathrm{Sn}^{+2}\right]}{\left\lceil\mathrm{Pb}^{+2}\right\rceil}=2.15$
$H_3PO_4 + OH^- \rightarrow H_2PO_4^- + H_2O$ ;
$H_3PO_4 + 2OH^- \rightarrow HPO_4^{2-} + 2H_2O$ ;
$H_3PO_4 + 3OH^- \rightarrow PO_4^{3-} + 3H_2O$