Question

Volatile liquids A and B forming an azeotrope (at a particular temperature range) have vapour pressures of $280mm$ and $200mm$ of $Hg$ respectively. If the mole fraction of A in the solution mixture is 0.3, and the mixture has a total vapour pressure of $300mm$ of $Hg$.

Choose the correct statement(s) for the following mixture.

• A. It forms a minimum boiling azeotrope.
• B. First minimum and then maximum boiling azeotrope is formed.
• C. It forms a maximum boiling azeotrope.
• D. First maximum and then minimum boiling azeotrope is formed.

Answer 1

The correct option is A It forms a minimum boiling azeotrope.

Given,
$\text{Pure vapour pressure of A ,}{p}_A^{\circ }=280mm\text{Pure vapour pressure of B ,}{p}_A^{\circ }=200mm$

$\text{Mole fraction of A ,}{X}_A=0.3\therefore \text{Mole fraction of B,}{X}_B=1-0.3=0.7$

By Raoult's law
Total vapour pressure of solution,
$P_T=X_A.p_A^{\circ }+X_B.p_B^{\circ }$
$P_T=280\times 0.3+200\times 0.7$
$=224mm.$

$P_T>X_A.p_A^{\circ }+X_B.p_B^{\circ }$
$300mm>224mm$
Thus, the given mixture shows positive deviation from the Raoult's law and forms minimum boiling azeotrope