Chapter 4, Problem 4.162QP

A quantity of 25.0 mL of a solution containing both Fe²⁺ and Fe³⁺ ions is titrated with 23.0 mL of 0.0200 M KMnO₄ (in dilute sulfuric acid). As a result, all of the Fe²⁺ ions are oxidized to Fe³⁺ ions. Next, the solution is treated with Zn metal to convert all of the Fe³⁺ ions to Fe²⁺ ions. Finally, the solution containing only the Fe²⁺ ions requires 40.0 mL of the same KMnO₄ solution for oxidation to Fe³⁺. Calculate the molar concentrations of Fe²⁺ and Fe³⁺ in the original solution. The net ionic equation is

${\text{MnO}}_4^{-}+5{\text{Fe}}^{2+}+8{\text{H}}^{+}\to {\text{Mn}}^{2+}+5{\text{Fe}}^{3+}+4{\text{H}}_2\text{O}$