The correct option is C 120.66 JK−1 mol−1
We are given that,
△Hvap=42.4 kJmol−1,T=273+78.4=351.4K
We know that,
△Svap=△HvapT
putting the values,
=42.4×103351.4=120.66 JK−1mol−1
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.