When 10 mL of an aqueous solution of Fe²⁺ ions was titrated in the presence of dil H₂SO₄ using diphenylamine indicator, 15 mL of 0.02 M solution of K₂Cr₂O₇ was required to get the end point. The molarity of the solution containing Fe²⁺ ions is x × 10⁻² M. The value of x is __________. (Nearest integer)

Solution

Chemistry ma’am · 4 steps

Match electrons, not moles: one dichromate takes six electrons, one Fe²⁺ gives just one.

1. Electron count. In acid, Cr₂O₇²⁻ goes to 2Cr³⁺. Each Cr drops from +6 to +3, so one dichromate ion takes 6 electrons. Each Fe²⁺ goes to Fe³⁺ and gives 1 electron.

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

2. Moles of dichromate. 15 mL × 0.02 M = 0.3 mmol of Cr₂O₇²⁻. By the 1 : 6 ratio, this reacts with 6 × 0.3 = 1.8 mmol of Fe²⁺.
3. Molarity of iron. These 1.8 mmol were in 10 mL, so M = 1.8/10 = 0.18 M = 18 × 10⁻² M.
4. So. The value of x is 18.

The common slip: 3. Treating dichromate and Fe²⁺ as reacting 1 : 1 drops the factor of 6 and gives M = 0.03 M, so x = 3.

The idea to keep. In a redox titration, equate milliequivalents: n₁ M₁ V₁ = n₂ M₂ V₂. For acidified dichromate the n-factor is 6, for Fe²⁺ it is 1.

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