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Question

Ethanol boils at 78.4oC and the enthalpy of vaporisation of ethanol is 42.4 kJ mol1. Calculate the entropy of vaporisation of ethanol.

A
101.5 JK1 mol1
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B
150 JK1 mol1
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C
120.66 JK1 mol1
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D
135 JK1 mol1
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Solution

The correct option is C 120.66 JK1 mol1
We are given that,
Hvap=42.4 kJmol1,T=273+78.4=351.4K

We know that,
Svap=HvapT
putting the values,
=42.4×103351.4=120.66 JK1mol1


Theory:

Entropy Calculation with phase transformation:
Phase change occurs at constant Pressure & Temperature and is considered reversible if it occurs at its transition temperature.

Entropy change for Vaporisation:
At constant pressure, q=qp = ΔH
ΔSvap = ΔHvapTb where Tb is boiling point of a substance.

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