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Question

Calculate the entropy change (in JK1 mol 1) for vaporisation of liquid water to steam at 100C. Given that heat of vaporisation is 40.8 kJmol1.

A
109.38
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B
100.38
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C
120.38
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D
129.38
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Solution

The correct option is A 109.38
We know that,
ΔS=ΔHvT
Here given that,
ΔH=40.8 kJ/mol, T=373K
putting the values,
ΔS=40.8×103373=109.38 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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