The density of a $3M$ sodium thiosulphate $\left({\mathrm{Na}}_2{\mathrm{S}}_2{\mathrm{O}}_3\right)$ solution is $1.25 g I mL$ Calculate the percent by weight of sodium thiosulphate

Mass of $1000 mL$ of ${\mathrm{Na}}_2{\mathrm{S}}_2{\mathrm{O}}_3$ solution $=1.25\times 1000=1250\mathrm{g}$ 

$\begin{array}{l}\text{ Molarity }=\frac{\text{ mass }}{\text{ molecular mass }\times \text{ volume }\left(mL\right)}\times 1000\\ \Rightarrow 3=\frac{\text{ mass }\times 1000}{158\times 1000}\end{array}$

$\therefore$ Mass of ${\mathrm{Na}}_2{\mathrm{S}}_2{\mathrm{O}}_3$ is $1000 mL$ of $3M$ solution 

$=3\times 158=474\mathrm{g}$

Percentage by weight of ${\mathrm{Na}}_2{\mathrm{S}}_2{\mathrm{O}}_3$ in solution

$=\frac{474}{1250}\times 100=37.92\mathrm{\%}$