Question

The density of $3M$ solution of $N{a}_2{S}_2{O}_3$ is $1.25gm{L}^{-1}$. Calculate, the molalities of $N{a}^{+}$ and $S_2{O}_3^{2-}$ ions respectively.

• A. 7.732 and 3.865
• B. 3.865 and 7.732
• C. 5.643 and 4.321
• D. None of the above

Answer 1

The correct option is A 7.732 and 3.865

Given, $\text{Molarity of}N{a}_2{S}_2{O}_3=3mol{L}^{-1}$
$\therefore$ Moles of $N{a}_2{S}_2{O}_3=3$
$\therefore \text{Weight of}N{a}_2{S}_2{O}_3=3\times 158=474g$
$\text{and Volume of solution}=1L=1000mL$
$\therefore \text{Weight of solution}=1000\times 1.25=1250g$
$\therefore \text{Weight of water}=1250-474=776g$

$\text{Molality of}N{a}^{+}=\frac{\text{Moles of}N{a}^{+}}{\text{Weight of water (g)}}\times 1000=\frac{6\times 1000}{776}=7.732$
Molality of $S_2{O}_3^{2-}=\frac{3\times 1000}{776}=3.865$