During the discharge of a lead storage battery, the density of sulphuric acid fell down from 1.294 to 1.139 g/ml. Sulphuric acid of density 1.294g/ml is 39% `H_2SO_4` by weight and that of density of 1.139 g/ml is 20% `H_2SO_4` by weight. The battery holds 3.5 L of acid and the volume remains practically constant during discharge. calculate the number of ampere hours for which the battery must have been used. The charging and discharging reactions are:
`Pb^(2+)+SO_4^(2-)LeftrightarrowPbSO_4+2bare(Charging)`
`PbO_2+4H^(+)+SO_4^(2-)+2bareLeftrightarrowPbSO_4+2H2O(Discharging)`.