A zinc cell runs at 8000 A for 24 h with a voltage of 3.35 V across it…

Step 1. The charge. $Q = 8000 \times 24 \times 3600 = 6.912 \times 10^{8}\ \text{C}$.

Step 2. The zinc. If all the charge made zinc: $\dfrac{6.912 \times 10^{8}}{2 \times 96500} \times 65.4 = 234\ \text{kg}$. At $91$ per cent: $0.91 \times 234 = 213\ \text{kg}$.

Step 3. The energy. $VQ = 3.35 \times 6.912 \times 10^{8} = 2.315 \times 10^{9}\ \text{J}$, which is $\dfrac{2.315 \times 10^{9}}{3.6 \times 10^{6}} = 643\ \text{kWh}$. For each kilogram, $\dfrac{643}{213} = 3.02\ \text{kWh/kg}$.

Step 4. The hydrogen. $9$ per cent of the charge: $\dfrac{0.09 \times 6.912 \times 10^{8}}{2 \times 96500} = 322\ \text{mol}$. At $22.4\ \text{L/mol}$ that is $7.2\ \text{m}^3$.