Question

250 mL of a waste solution obtained from the workshop of a goldsmith contains 0.1 M AgNO₃ and 0.1 M AuCl. The solution was electrolyzed at 2 V by passing a current of 1 A for 15 minutes. The metal/metals electrodeposited will be:

• A. Silver and gold in proportion to their atomic weights.
• B. Silver and gold in equal mass proportion.
• C. only silver.
• D. only gold.

Answer 1

The correct option is A

Silver and gold in proportion to their atomic weights.

Faraday's - Second Law of electrolysis:

According to this law, if an equal amount of current is passed through an electrolytic solution containing more than one electrolyte, then the amount of substance deposited over the cathode is in the proportion of their equivalent weight.

Equivalent weight:

It is defined as the ratio of the atomic weight of the metal to the number of electrons required for the reduction of metal cation.

Explanation:

1. Gold$\left(\mathrm{Au}\right)$ and silver$\left(\mathrm{Ag}\right)$ both are univalent in their given compounds that is ${\mathrm{Au}}^{+}\mathrm{and}{\mathrm{Ag}}^{+}$, so both require one electron for their reduction.
2. So, for both gold and silver, atomic weight = equivalent weight.
3. 2V is applied across the solution containing ${\mathrm{Au}}^{+}$ and ${\mathrm{Ag}}^{+}$, both metals have redox potential less than 2V(as given), so both the metals will get reduced.
4. Since an equal amount of current is passed through the solution containing silver and gold salts, both the metals are univalent and have the same concentration.
5. Thus, they are deposited in the proportion of their atomic weights.