At STP, the molar volume of any gas is $22.4 \text{ L}$. The reaction for the production of hydrogen gas at the cathode is: $$2\mathrm{H}^+ + 2\mathrm{e}^- \rightarrow \mathrm{H}_2$$ From the reaction, $2 \text{ moles of electrons}$ (i.e., $2 \text{ F}$ charge) produce $1 \text{ mole}$ of $\mathrm{H}_2$ gas. Given volume of $\mathrm{H}_2$ gas, $V = 500 \text{ cm}^3 = 0.5 \text{ L}$. Number of moles of $\mathrm{H}_2$ gas produced: $$ \begin{aligned} n &= \frac{V}{22.4 \text{ L/mol}} \\ &= \frac{0.5}{22.4} \text{ mol} \\ &\approx 0.02232 \text{ mol} \end{aligned} $$ Since $1 \text{ mole}$ of $\mathrm{H}_2$ requires $2 \text{ F}$ of electricity, the charge $Q$ required is: $$ \begin{aligned} Q &= n \times 2 \times 96500 \text{ C} \\ &= 0.02232 \times 2 \times 96500 \text{ C} \\ &= 4307.76 \text{ C} \end{aligned} $$ Thus, the required electricity is approximately $4307.76 \text{ Coulombs}$.