Comprehension:
Electrolysis
In the electrolysis process, the nature of material being electrolyzed, and the electrodes nature are the important factors in predicting the products of electrolysis. The electrode material, whether acting as a source or sink or itself, is not inert will also decides the nature of the product of the electrolysis. The standard electrode potentials of the various oxidizing and reducing species present in the electrolytic mixture also decides the product of electrolysis. The voltage applied also decides the product of electrolysis since some of the reactions which are feasible are so slow kinetically that they are not seem to take place at lower voltage. The quantitative aspects of electrolysis was described by Faraday law of electrolysis.
How much charge is required to deposit 21 g of Copper from a solution of CuSO4?
(atomic mass of Cu = 63 g mol−1).
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 64333 C
By Faraday’s laws, the charge needed to deposit a metal depends on the number of electrons in its electrode reaction. One mole of electrons carries 1 faraday (1 F = 96500 C mol−1).
Copper is deposited from \( \ce{CuSO4} \) solution as:
\( \displaystyle \ce{Cu^2+(aq) + 2e^- -> Cu(s)} \)
So 2 F (2 × 96500 = 193000 C) deposit 1 mol (63 g) of copper.
\( \displaystyle n(\ce{Cu})=\frac{21}{63}=\frac{1}{3}\ \mathrm{mol} \)
\( \displaystyle Q=\frac{1}{3}\times2\times96500 \)
\( \displaystyle Q=\frac{193000}{3}=64333\ \mathrm{C} \)
So the charge required is 64333 C.
Shortcut: 21 g is one third of the molar mass of copper, so the charge is one third of 193000 C.