Question

$32g$ of a sample of $FeS{O}_4\cdot 7H_{2}O$ were dissolved in dilute sulphuric acid and water and its volume was made up to 1litre. $25mL$ of this solution required $20mL$ of $0.02MKMn{O}_4$ solution for complete oxidation. Calculate the mass $\%$ of $FeS{O}_4\cdot 7H_{2}O$ in the sample.

• A. $34.75$
• B. $69.5$
• C. $89.5$
• D. None of these

Answer 1

Answer: (B)

$Mn{O}_4+5F{e}^{2+}+8H^{+}\to M{n}^{2+}+5F{e}^{3+}+4H_{2}O$
$n_f\left(KMn{O}_4\right)=7-2=5$
$n_f\left(FeS{O}_4\right)=3-2=1$
$m$-eq. of $FeS{O}_4\cdot 7H_{2}O$ in 1 litre $=m$ - eq. of $KMn{O}_4$
$\Rightarrow 20\times 0.02\times 5\times \left(\frac{1000}{25}\right)=\frac{W}{278}\times 1\times 1000$
[Mass of $FeS{O}_4\cdot 7H_{2}O=278g/mol]$
$\Rightarrow W=22.24$
Mass $\%$ of $FeS{O}_4\cdot 7H_{2}O$ in given sample
$=\frac{22.24}{32}\times 100=69.5$