Question

Two liquids A and B are present in equimolar amount in a solution having vapour pressure $80\text{mm of Hg}$ for A and $120\text{mm of Hg}$ for B . The pressure of the solution is observed to be $90\text{mm of Hg}$. It shows that the solution exhibits:

• A. Positive deviation from ideal behaviour.
• B. Negative deviation from ideal behaviour.
• C. Ideal behaviour
• D. Positive deviation at low concentrations and negative at higher concentrations

Answer 1

The correct option is B Negative deviation from ideal behaviour.

From Raoults law,
$P_{\text{total}}=p_A^0\times X_A+p_B^0\times X_B$
$=80.0\times 0.5+120.0\times 0.5$
$=100\text{mm of Hg}$
The observed $P_{\text{total}}$ is $90\text{mm of Hg}$ which is less than $100\text{mm of Hg}$. Hence the solution shows negative deviation.