Question

# 0.15g of a substance dissolved in 15g of a solvent boiled at a temp. higher by 0.2160C then that of the pure solvent. Find out the molecular weight of the substance. [kb for solvent is 2.16oC]

A
1.01
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B
10.1
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C
100
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D
10
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Solution

## The correct option is C 100Weight of substance = 0.15gWeight of solvent = 15g = 0.015 kgm = molalityAs we know that,molality (m) = moles of soluteweight of solvent (in Kg)Let the molecular weight of substance dissolved be x.∴m=(weight of substancemol. weight of substance)weight of solvent (in Kg)=(0.15x)0.015=10xAs we know that,ΔTb=kb.mwhere,ΔTb = Elevation in boiling point = 0.216℃kb=2.16℃m = molality = 10x∴0.216=2.16×10x⇒x=100Hence, the molecular weight of the substance is 100 g.Hence, the correct option is C

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