Question

The density of 3M solution of $N{a}_2{S}_2{O}_3$ is $1.25$ g mL${}^{-1}$. Calculate, the $\%$ by weight of $N{a}_2{S}_2{O}_3$.

• A. 38%
• B. 50%
• C. 11%
• D. 98%

Answer 1

The correct option is A 38%

Given $N{a}_2{S}_2{O}_3\text{has molarity}=3mol{L}^{-1}$
$\therefore$ Mole of $N{a}_2{S}_2{O}_3=3$
$\therefore \text{Weight of}N{a}_2{S}_2{O}_3=3\times 158=474g$
$\text{and Volume of solution =1 litre = 1000 mL}$
$\therefore \text{Weight of solution}=1000\times 1.25=1250g$
$\therefore \text{Weight of water}=1250-474=776g$
$\text{\% by weight of}N{a}_2{S}_2{O}_3=\frac{\text{wt. of}N{a}_2{S}_2{O}_3}{\text{wt. of}N{a}_2{S}_2{O}_3+\text{wt. of}{H}_{2}O}\times 100$


$\%\text{by weight of}N{a}_2{S}_2{O}_3=0.3792\times 100=37.92$