Question

Total vapour pressure of mixture of $1$ mole of $A(P_A^0=150mm)$ and 2 mole of B $(P_B^0=240mm)$ is $200$mm. In this case :

• A. There is a positive deviation from Raoult's law.
• B. There is a negative deviation from Raoult's law.
• C. There is no deviation from Raoult's law.
• D. Molecular masses of A & B are also required for calculating deviation.

Answer 1

The correct option is A There is a positive deviation from Raoult's law.

Given moles of X = 1 mole ,Moles of Y = 2 moles

Partial pressure of X $=P_X^0\times x_X=150torr\times \frac{1}{1+2}=150\times 1/3torr$

partial pressure of Y$=p_X^0=240torr\times \frac{2}{1+2}=240\times 2/3torr$

so, the total pressure is = $p_X^0{x}_X+P^0{x}_Y$

$=150\times 1/3+240\times 2/3$

$=50+60$

$=210torr$

given pressure of the solution is $200mm$

but the calculated vapour pressure is above than given presssure is so mixture shows positive solution