20 mL of 0.02 M K₂ Cr₂ O₇ solution is used for the titration of 10 mL of Fe²⁺ solution in the acidic medium.
The molarity of Fe²⁺ solution is __________ × 10⁻² M. (Nearest Integer)

Solution

Chemistry ma’am · 4 steps

One dichromate takes six electrons and each iron gives one, so count electrons, not volumes.

1. Electrons per ion. In acid, each Cr goes from +6 to +3, so one Cr₂O₇²⁻ gains 6 electrons. Each Fe²⁺ loses 1 electron to become Fe³⁺. So 1 mol dichromate reacts with 6 mol Fe²⁺.

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

2. Moles of dichromate. 20 mL × 0.02 M = 0.4 mmol of K₂Cr₂O₇. That reacts with 6 × 0.4 = 2.4 mmol of Fe²⁺.
3. Molarity of iron. M = (2.4 mmol)/(10 mL) = 0.24 M = 24 × 10⁻² M.
4. So. The molarity of the Fe²⁺ solution is 24 × 10⁻² M.

The common slip: 4. Using M₁V₁ = M₂V₂ without the factor of 6 gives 0.04 M, that is 4, not 24.

The idea to keep. In a redox titration, equate milliequivalents: M₁ n₁ V₁ = M₂ n₂ V₂, where n is electrons per formula unit. For dichromate in acid, n = 6.

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