Redox Reactions

Worked Chemistry Problems

Redox reactions involve charge as well as mass.
Redox reactions involve charge as well as mass. Rafe Swan, Getty Images

This is a worked example redox reaction problem showing how to calculate volume and concentration of reactants and products using a balanced redox equation.

Redox Reaction Problem

Given the following balanced redox equation for the reaction between MnO4- and Fe2+ in an acidic solution:

MnO4-(aq) + 5 Fe2+(aq) + 8 H+(aq) → Mn2+(aq) + 5 Fe3+(aq) + 4 H2O

Calculate the volume of 0.100 M KMnO4 needed to react with 25.0 cm3 0.100 M Fe2+ and the concentration of Fe2+ in a solution if you know that 20.0 cm3 of solution reacts with 18.0 cm3 of 0.100 KMnO4.

How to Solve

Since the redox equation is balanced, 1 mol of MnO4- reacts with 5 mol of Fe2+. Using this, we can obtain the number of moles of Fe2+:

moles Fe2+ = 0.100 mol/L x 0.0250 L

moles Fe2+ = 2.50 x 10-3 mol

Using this value:

moles MnO4- = 2.50 x 10-3 mol Fe2+ x (1 mol MnO4-/ 5 mol Fe2+)

moles MnO4- = 5.00 x 10-4 mol MnO4-

volume of 0.100 M KMnO4 = (5.00 x 10-4 mol) / (1.00 x 10-1 mol/L)

volume of 0.100 M KMnO4 = 5.00 x 10-3 L = 5.00 cm3

To obtain the concentration of Fe2+ asked in the second part of this question, the problem is worked the same way except solving for the unknown iron ion concentration:

moles MnO4- = 0.100 mol/L x 0.180 L

moles MnO4- = 1.80 x 10-3 mol

moles Fe2+ = (1.80 x 10-3 mol MnO4-) x (5 mol Fe2+ / 1 mol MnO4)

moles Fe2+ = 9.00 x 10-3 mol Fe2+

concentration Fe2+ = (9.00 x 10-3 mol Fe2+) / (2.00 x 10-2 L)

concentration Fe2+ = 0.450 M