The resistance of $$0.1 \mathrm{~M}$$ weak acid $$\mathrm{H} A$$ in a conductivity cell is $$2 \times 10^3 \mathrm{~Ohm}$$. The cell constant of the cell is $$0.78 ~\mathrm{C} \mathrm{~m}^{-1}$$ and $$\lambda_{\mathrm{m}}^{\circ}$$ of acid $$\mathrm{H} A$$ is $$390 \mathrm{~S} \mathrm{~cm}^2 \mathrm{~mol}^{-1}$$. The $$\mathrm{pH}$$ of the solution is

During the electrolysis of brine, by using inert electrodes,

Consider the following 4 electrodes

$$\begin{aligned} & \mathrm{A}: \mathrm{Ag}^{+}(0.0001 \mathrm{M}) / \mathrm{Ag}(s) ; \\ & \mathrm{B}: \mathrm{Ag}^{+}(0.1 \mathrm{M}) / \mathrm{Ag}(s) ; \\ & \mathrm{C}: \mathrm{Ag}^{+}(0.01 \mathrm{M}) / \mathrm{Ag}(s) ; \\ & \mathrm{D}: \mathrm{Ag}^{+}(0.001 \mathrm{M}) / \mathrm{Ag}(s) ; E^{\circ}{ }_{\mathrm{Ag}^{+} / \mathrm{Ag}}=+0.80 \mathrm{~V} \end{aligned}$$

Then reduction potential in volts of the electrodes in the order.

In fuel cells _______ are used as catalysts.