Consider the reaction of ${\text{KMnO}}_4$ with ${\text{H}}_2{\text{O}}_2$ in acidic medium. If $10\text{ mL}$ of $0.1\text{ M }{\text{KMnO}}_4$ reacts completely with $10\text{ mL}$ of ${\text{H}}_2{\text{O}}_2$, what is the molarity of the ${\text{H}}_2{\text{O}}_2$ solution?

Solution

1. The balanced redox reaction is $2{\text{MnO}}_4^{-}+5{\text{H}}_2{\text{O}}_2+6{\text{H}}^{+}\to 2{\text{Mn}}^{2+}+5{\text{O}}_2+8{\text{H}}_2\text{O}$.
2. The moles of ${\text{KMnO}}_4$ used are $$\begin{array}{rl}n& =0.1\text{ M}\times 0.01\text{ L}\\ & =0.001\text{ mol}\end{array}$$.
3. From stoichiometry, moles of $$\begin{array}{rl}{\text{H}}_2{\text{O}}_2& =\frac{5}{2}\times 0.001\\ & =0.0025\text{ mol}\end{array}$$.
4. Molarity of $$\begin{array}{rl}{\text{H}}_2{\text{O}}_2& =\frac{0.0025\text{ mol}}{0.01\text{ L}}\\ & =0.25\text{ M}\end{array}$$.
5. Hence the answer is (B).