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Question

If the density of a 3 M thiosulphate (Na2S2O3) solution is 1.2 g/mL, then the molality of the solution is:

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
= 1200 - 474 = 726 g
Molality (m) = number of moles of solutemass of solvent in g×1000
Molality = 3726×1000 = 4.13 m

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