Question

At ${55}^{0}C$, ethanol has a vapour pressure of $168$ mm Hg, and the vapour pressure of methylcyclohexane is $280$ mm Hg. A solution of the two in which the mole fraction of ethanol is $0.68$, has a total vapour pressure of $225$ mm Hg. This solution is formed from its components with:

• A. the evolution of heat
• B. the absorption of heat
• C. neither absorption nor evolution of heat
• D. nothing can be predicted on the basis of given information

Answer 1

The correct option is B the absorption of heat

First, we will calculate total vapour pressure assuming ideal behaviour.
$P_T=P_A^0{X}_A+P_B^0{X}_B$

$P_T=168\times 0.68+280\times (1-0.68)=203.84$ mm Hg.
This is lower than the total vapour pressure of $225$ mm Hg.

Hence, the non-ideal solution shows a positive deviation from Raoult's law. For such a solution, the enthalpy change is positive. Hence, this solution is formed from its components with the absorption of heat.