$Al _{2} O _{3}$ is reduced by electrolysis at low potentials and high currents. If $4.0 \times 10^{4}$ amperes of current is passed through molten $Al _{2} O _{3}$ for 6 hours, what mass of aluminium is produced ? (Assume $100 \%$ current efficiency, at. mass of $Al =27\, g\, mol ^{-1}$ ) -