Question

If the density of a 3 M thiosulphate $\left(N{a}_2{S}_2{O}_3\right)$ solution is 1.2 g/mL, then the molality of the solution is:

• A. 4.13 m
• B. 2.57 m
• C. 3.85 m
• D. 5.46 m

Answer 1

The correct option is A 4.13 m

3 M solution means 3 moles of sodium thiosulphate is present in 1 L of the solution.
Mass of ($N{a}_2{S}_2{O}_3$) in solution = $3\times molarmass$
Molar mass of ($N{a}_2{S}_2{O}_3$) = 158 g/mol
Mass of solute = 3 $\times$ 158 = 474 g
Since, Density = $\frac{mass}{volume}$
Mass of solution = 1.2 g/mL $\times$ 1000 mL = 1200 g
Mass of solvent = mass of solution - mass of solute
= 1200 - 474 = 726 g
Molality (m) = $\frac{\text{number of moles of solute}}{\text{mass of solvent in g}}\times 1000$
Molality = $\frac{3}{726}\times 1000$ = 4.13 m