Question

$1.355g$ of pyrolusite sample are added to $50mL$ of $1N$ oxalic acid solution containing sulphuric acid. After the reaction is completed, the contents are transferred to a measuring flack and the volume made up to $200mL$. $20mL$ of this solution is titrated against $KMn{O}_4$ solution whose strength is $2g/L$ and $31.6mL$ of $KMn{O}_4$ solution are required. Calculate the percentage purity in the given sample of pyrolusite.

Answer 1

$20mL$ of undecomposed oxalic acid requires = $31.6mL,\frac{2}{31.6}N$ of $KMn{O}_4$ solution

Thus $200mL$ of undecomposed oxalic acid requires = $\frac{31.6\times 200}{20}=316mL,\frac{2}{31.6}N$ of $KMn{O}_4$ solution

using $N_1\times V_1=N_2\times V_2$

= $316mL\times \frac{2}{31.6}N=xmL\times 1N$

= x mL = 30 mL

$\therefore 30mLof1NMn{O}_{2}solution$

Mass of $Mn{O}_2$ = $\frac{N\times E\times V}{1000}=\frac{1\times 87\times 30}{2\times 1000}=1.305gm$

percentage purity of pyrolusite = $\frac{1.305\times 100}{1.355}=96.31\%$