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Question

A solution M is prepared by mixing ethanol and water. The mole fraction of ethanol in the mixture is 0.9. Water is added to the solution M such that the fraction of water in the solution becomes 0.9. The boiling point of this solution is :

A
380.4 K
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B
376.2 K
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C
375.5 K
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D
354.7 K
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Solution

The correct option is B 376.2 K
The expression for the elevation in the boiling point of the solution is
ΔTb=Kb×m......(i)

Here, ΔTb is the elevation in the boiling point, Kb is the boiling point elevation constant of water
and m is the molality of the solution.

The mole fraction of water is 0.9 and the mole fraction of ethanol is 0.1
Thus water acts as a solvent.

The molality of a solution is the number of moles of ethanol (solute) present in 1 kg of water (solvent).
Molar mass of water = 18 gram/mole.

Hence, the molality of the solution is
m=0.10.9×18×1000 m

Substitute values in equation (i)
The elevation in boiling point is ΔTb=0.52×0.10.9×18×1000=3.2K

The boiling point of pure water is 373 K.

Boiling point of solution =373+3.2=376.2K

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