Question

20 mL of 0.02 M K₂Cr₂O₇ 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)

Answer 1

Correct Answer: 24

The titration equation for the reaction between dichromate ions (Cr₂O₇^2–) and iron ions (Fe²⁺) in an acidic medium is:
Cr₂O₇^2– + 6Fe²⁺ + 14H⁺ → 2Cr³⁺ + 6Fe³⁺ + 7H₂O
The stoichiometry indicates that 1 mole of Cr₂O₇^2– oxidizes 6 moles of Fe²⁺.

Calculate moles of K₂Cr₂O₇: 
 moles = Molarity × Volume (L) = 0.02 M × 0.020 L = 0.0004 mol

Since 1 mole of Cr₂O₇^2– reacts with 6 moles of Fe²⁺, moles of Fe²⁺ are:
 6 × 0.0004 mol = 0.0024 mol

The molarity of Fe²⁺ solution is calculated using its volume:
 Molarity = (moles/Volume in L) = 0.0024 mol / 0.010 L = 0.24 M

Express it as: 24 × 10^–2 M.
The solution value is 24, which lies within the expected range (24,24). Thus, the molarity of the Fe²⁺ solution is 24×10^–2 M.

Answer 2

Applying the law of equivalence,
milliequivalents of Fe²⁺ = milliequivalents of K₂Cr₂O₇
10 × 1 × M = 20 × 6 × .02
M = 24 × 10^–2 M
∴ Answer will be 24.