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Question

Find the molality of the thiosulphate (Na2S2O3) solution, if the density of a 3 M thiosulphate (Na2S2O3) solution is 1.2 g/mL.

A
4.13 m
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B
2.57 m
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C
3.85 m
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D
5.46 m
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Solution

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 (Na2S2O3) in solution = 3×molar mass
Molar mass of (Na2S2O3=158 g/mol)
Mass of solute = 3×158=474 g
Since, Density = massvolume
Mass of solution =1.2 g/mL×1000 mL=1200 g
Mass of solvent = mass of solution - mass of solute
=1200474=726 g
Molality(m)=number of moles of solutemass of solvent in kg×1000
Molality=3726×1000=4.13 m

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