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Question

On dissolving 10.8 g glucose (molar weight = 180 g/mol) in 240 g of water, its boiling point increases by 0.13C. Calculate the molal elevation constant of water.

A
0.75C kg mol1
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B
0.52C kg mol1
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C
0.45C kg mol1
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D
1C kg mol1
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Solution

The correct option is B 0.52C kg mol1
Boiling point elevation of a solution is given by,
Tb=Kb×m
where,
Tb is the elevation in boiling point.
Kb is molal elevation constant.
m is molality of solution.

Molality (m)=nsolute×1000Wsolventin gram
Molality (m)=wsolute×1000Msolute.Wsolventin gram
where,
wsolute is mass of solute
Wsolvent is mass of solvent
Msolute is molar mass of solute

Tb=1000Kb×wW×Mor Kb=Tb×W×M1000×wGiven, Tb=0.13C,W=240 g,M=180 and w=10.8 gKb=0.13×240×1801000×10.8=0.52C kg mol1

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