15 mL of aqueous solution of Fe²⁺ in acidic medium completely reacted with 20 mL of 0.03 M aqueous Cr₂O₇²⁻. The molarity of the Fe²⁺ solution is __________ × 10⁻² M. (Round off to the Nearest Integer).

Solution

Chemistry ma’am · 4 steps

One dichromate ion takes six electrons, so it oxidises six Fe²⁺ ions. Use that 1 : 6 ratio.

1. Balance the reaction. In Cr₂O₇²⁻ each Cr goes from +6 to +3, so one ion gains 2 × 3 = 6 electrons. Each Fe²⁺ loses only 1 electron, so six of them are needed for one dichromate ion.

   Cr₂O₇²⁻ + 6Fe²⁺ + 14H⁺ → 2Cr³⁺ + 6Fe³⁺ + 7H₂O

2. Moles of dichromate. 0.03 M × 0.020 L = 6 × 10⁻⁴ mol of Cr₂O₇²⁻.
3. Moles of iron. Fe²⁺ needed = 6 × 6 × 10⁻⁴ = 3.6 × 10⁻³ mol. These are in 15 mL, so the molarity is (3.6 × 10⁻³)/0.015 = 0.24 M.
4. So. 0.24 M = 24 × 10⁻² M, so the answer is 24.

The common slip: 4. Taking the Fe²⁺ to Cr₂O₇²⁻ ratio as 1 : 1 instead of 6 : 1 gives 0.04 M, which is 4, not 24.

The idea to keep. In redox titrations, the electrons lost must equal the electrons gained. Find each species' electron change first, then let that ratio set the moles.

Answer from the NTA final answer key. Solution by PrepWiser.