Question

1. Consider the reaction: Cr2O72- (aq) + 14 H+ (aq) + 6 Fe2+ (aq) >>> 2 Cr3+ (aq) + 6 Fe3+ (aq) + 7 H2O (l)

In the lab, it is found that 19.30 mL of K2Cr2O7 solution is needed to react with 25.00 mL of 0.4000 M Fe2+ (aq) solution, what is the molarity of the potassium dichromate solution?

Answer 1

Reaction

Cr2O72- (aq) + 14 H+ (aq) + 6 Fe2+ (aq) >>> 2 Cr3+ (aq) + 6 Fe3+ (aq) + 7 H2O (l)

Volume of K2Cr2O7 (aq) = 19.30 mL = 0.0193 L

Volume of Fe²⁺(aq) = 25.0 mL = 0.025 L

Molarity of Fe²⁺ = 0.4000 M

Calculation of moles of Fe²⁺

n Fe²⁺ = volume in L x molarity

= 0.025 L x 0.4000 M

= 0.01 mol

Calculation of moles of potassium dichromate

n potassium dichromate = moles of Fe²⁺ x 1 mol postassium dichromate / 6 mol Fe²⁺

= 0.01 mol Fe²⁺ x 1 mol potassium dichromate / 6 mol Fe²⁺

= 0.00167 mol potassium dichromate

Molarity = mol / volume in L

= 0.00167 mol / 0.0193 L

= 0.0864 M

Molarity of potassium dichromate = 0.0864 M