Question

A sample of $H_2{O}_2$ labelled as $25\%$ (w/w) has a density of $1.25\text{g/mL}$. Which among the following correctly represents its concentration?
i. Molarity $=9.2\text{M}$
ii. Molality $=9.8\text{m}$
iii. $\%$ w/V $=33.6\%$
iv. ppm (w/w) $=2500\text{ppm}$

• A. i, ii
• B. i, ii, iii
• C. ii, iii, iv
• D. i, iii

Answer 1

The correct option is A i, ii

$25\%$ (w/w) $H_2{O}_2$ = $25$ g $H_2{O}_2$ in $100$ g solution
$\Rightarrow$ $250$ g $H_2{O}_2$ will be present in $1000$ g solution

$\text{Volume of the solution}=\frac{\text{mass}}{\text{density}}=\frac{1000}{1.25}=800\text{mL}$

$\text{Molarity}=\frac{\text{No. of moles of solute}}{\text{Volume of solution in L}}=\frac{250}{34}\times \frac{1000}{800}=9.2\text{M}$

$\text{Molality}=\frac{\text{No. of moles of solute}}{\text{Mass of solvent in kg}}=\frac{250}{34}\times \frac{1000}{750}=9.8\text{m}$

$\Rightarrow \%\frac{w}{v}=\frac{250}{750}\times 100=33.3\%$

$\Rightarrow$ppm (w/w) concentration $=\frac{25}{100}\times {10}^6=2.5\times {10}^5\text{ppm}$