The density of a 3 molal solution of sodium thiosulphate $\left(\mathrm{Na}_{2} \mathrm{~S}_{2}…
The density of a 3 molal solution of sodium thiosulphate $\left(\mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3} \cdot 5 \mathrm{H}_{2} \mathrm{O}ight)$ solution is $1.25 \mathrm{~g} \mathrm{~cm}^{-3}$. The percentage by mass of sodium thiosulphate is
$35.25 \%$
$39.20 \%$
$42.67 \%$
$46.67 \%$
Solution
Molar mass of $\mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3} \cdot 5 \mathrm{H}_{2} \mathrm{O}$ is $\quad M=(2 \times 23+2 \times 32+3 \times 16+5 \times 18) \mathrm{g} \mathrm{mol}^{-1}=248 \mathrm{~g} \mathrm{~mol}^{-1}$ The mass of $3 \mathrm{~mol}$ of $\mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3} \cdot 5 \mathrm{H}_{2} \mathrm{O}$ is $m_{2}=(3 \mathrm{~mol})\left(248 \mathrm{~g} \mathrm{~mol}^{-1}ight)=744 \mathrm{~g}$
The mass of solution will be $m=m_{1}+m_{2}=1000 \mathrm{~g}+744 \mathrm{~g}=1744 \mathrm{~g}$
Mass percentage of sodium thiosulphate $=\frac{m_{2}}{m} \times 100=\left(\frac{744 \mathrm{~g}}{1744 \mathrm{~g}}ight) \times 100=42.67 \%$
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