Three electrolytic cells A, B and C containing solutions of ZnSO4(zinc sulphate), AgNO3 (silver nitrate) and CuSO4 (copper sulphate), respectively are connected in series. A steady current of 1.5 ampere was passed through them until 1.45 g of silver is deposited at the cathode of cell B. How long did the current flow? What mass of copper and zinc were deposited? (Atomic masses: Ag = 108, Zn = 65.4, Cu = 63.5, all in amu).
108 g of silver is deposited at cathode when 1 mol of electrons are passed or 108 g of silver deposit needs = 1 Faraday = 96,500 coulombs. Therefore, 1.45 g of silver needs
But quantity of electricity passed
= current x time
= 1295.6 C = 1.5 A x time (in sec.)
or time for which current is passed
The cathode reaction in copper sulphate cell is
2 x 96500 coulombs gives a deposit of 63.5 g of Cu.
Therefore, 1295.6 coulombs will deposit of
(cathode reaction in zinc sulphate cell)
Mass of zinc deposited