 Solution

ID: past-exam-of-the-physics-b-course-of-the-university-of-cambridge/2014/paper-2/section-a/a1/solution

Treat the vapour as an ideal gas, neglect the liquid's molar volume, and take the molar latent heat as constant across this small interval. Integrating the Clausius-Clapeyron equation gives

ln \frac{p _{2}}{p _{1}} = \frac{L _{m}}{R} (\frac{1}{T _{1}} - \frac{1}{T _{2}}), L_{m} = \frac{R ln ( 1.1 )}{1/230.7 - 1/233.4} ≃ 15.8 kJ mo l^{- 1} .

Only the pressure ratio enters, so conversion from atmospheres to pascals is unnecessary.

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