Question

The vapour pressure of two pure liquids $A$ and $B$ forming an ideal solution are 400 mm Hg and 800 mm Hg respectively at temperature $T$. A liquid mixture containing $3:1$ molar composition of $A$ and $B$ is present in a cylinder closed by a piston so that the pressure can be varied. The solution is slowly vaporised at temperature $T$ by decreasing the applied pressure from a starting pressure of 760 mm Hg. A pressure gauge (in mm Hg) is connected which gives the reading of pressure applied.

The reading of pressure gauge at bubble point is:

• A. $500$ mm of Hg
• B. $600$ mm of Hg
• C. $700$ mm of Hg
• D. None of the above

Answer 1

The correct option is A $500$ mm of Hg

$X_A=0.75,X_B=0.25$ ......As mixture is in 3:1 ratio

Above 500 mm Hg, only liquid phase exists.

$P_{bubblepoint}=X_A{P}_A^o+X_B{P}_B^o$

$=0.75\times 400+0.25\times 800=500mm$.

Thus the reading of pressure gauge at bubble point is 500 mm of Hg.

Hence, the correct option is $A$