(a) $\;t=900\ \mathrm{s}$ $\;\;\text{ Charge }\;=\;\text{ Current }\; \times \;\text{ Time }\;$ $=2 \times 900$ $\;=1800\ \mathrm{C}$ According to the reaction $\mathrm{Ag}^{+} \;\text{(aq) }\;+ \mathrm{e}^{-} \rightarrow \mathrm{Ag}(\mathrm{s})$ We require $1\ \mathrm{F}$ to deposit $1\ \mathrm{mol}$ or $108\ \mathrm{g}$ of $\mathrm{Ag}$ For $1800\ \mathrm{C}$, the mass of $\mathrm{Ag}$ deposited will $\;\text{ be }\;=\;\frac{108 \times 1800}{1 \times 96500}=2.0145\ \mathrm{g}$ (b) Fuel cell is the name given to the galvanic cells which are designed to convert the energy of combustion of fuels like hydrogen, methane, methanol, etc. directly into electrical energy.