This is a worked example redox reaction problem showing how to calculate volume and concentration of reactants and products using a balanced redox equation.
A redox reaction is a type of chemical reaction in which reduction and oxidation occur. Because electrons are transferred between chemical species, ions form. So, to balance a redox reaction requires not only balancing mass (number and type of atoms on each side of the equation), but also charge.
In other words, the number of positive and negative electrical charges on both sides of the reaction arrow are the same in a balanced equation.
Once the equation is balanced, the mole ratio may be used to determine the volume or concentration of any reactant or product as long as the volume and concentration of any species is known.
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.
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
When solving this type of problem, it’s important to check your work: