Question

What is the number of electrons gained by Cl atom when \(n\) mole of \(\text{ClO}_4^-\) is transformed into \(n\) mole of \(\text{ClO}_2^-\) ion?

• A. \(3n\)
• B. \(4n\)
• C. \(n\)
• D. \(2n\)

Hint:

Find the oxidation number of Cl in each ion and take the difference.

Answer 1

The Correct Option is B

Step 1: Approach
Balance the half reaction in acid by charges and see how many electrons appear.

Step 2: Equation
\[ \text{ClO}_4^- + 4\text{H}^+ + 4e^- \to \text{ClO}_2^- + 2\text{H}_2\text{O} \]

Step 3: Check the balance
Oxygen: 4 on the left, $2+2=4$ on the right. Hydrogen: 4 on the left, 4 on the right. Charge on the left: $-1+4-4=-1$. Charge on the right: $-1$. The equation is balanced with 4 electrons.

Step 4: Scale by n
For $n$ mol of reactant the electron gain is $4n$ mol, so (B) is correct.

Final Answer:
Each Cl atom goes from +7 to +3, gaining 4 electrons, so n mol gain 4n electrons, option (B). \[ \boxed{4n} \]