Silver and copper voltmeters are connected in parallel with a battery of emf 12 V. In 30 min 1 g of silver and 1.8 g of copper are liberated. The energy supplied by the battery is :

Number of moles of silver and copper deposited are 1/108 and 1.8/63 (according to mole concepts)
Number of electrons required to discharge Ag and Cu will be proportional to charges.
Hence total charge is 1/108 + 2 × 1.8/63.
All the charge is passed through 12V battery so total energy is = [1/108 + 2 × 1.8/63] × 12 × [Avogadro number]
Therefore total energy is 4.34 × 10^4 J