Question #99871

A solid mixture of $\ce{KMnO4}$ and $\ce{K2Cr2O7}$ is dissolved in water to prepare $200\text{ mL}$ of a stock solution.

Aliquot 1 ($100\text{ mL}$) of this solution is strongly acidified with dilute $\ce{H2SO4}$ and treated with an excess of aqueous $\ce{KI}$. The liberated iodine requires $160\text{ mL}$ of $0.1\text{ M } \ce{Na2S2O3}$ solution for complete titration.

Aliquot 2 ($100\text{ mL}$) is buffered to $\text{pH} = 9$. It is then titrated directly with $0.1\text{ M } \ce{Na2S2O3}$ solution, requiring $7.5\text{ mL}$ to reach the potentiometric end point.

Calculate the mass of $\ce{KMnO4}$ and $\ce{K2Cr2O7}$ in the original solid mixture. (Given molar masses: $\ce{KMnO4} = 158\text{ g/mol}$, $\ce{K2Cr2O7} = 294\text{ g/mol}$)