Components a and b form ideal solution. at 350 k, a liquid mixture containing 40% (mole) a is in equilibrium with a vapour containing 70% (mole) a. if the vapour pressure of a at 350 k is 70 kpa, what is the vapour pressure of b? (b) 20 kpa (d) 12 kpa (а) 25 kpa (c) 40 kpa

Answer : The correct option is, (b) 20 kPa

Explanation :

The Raoult's law for liquid phase is:

    .............(1)

where,

= partial vapor pressure of A

= vapor pressure of pure substance A

= mole fraction of A

The Raoult's law for vapor phase is:

     .............(2)

where,

= partial vapor pressure of A

= total pressure of the mixture

= mole fraction of A

Now comparing equation 1 and 2, we get:

   ............(3)

First we have to calculate the total pressure of the mixture.

Given:

and

and

Now put all the given values in equation 3, we get:

Now we have to calculate the vapor pressure of B.

Formula used :

Now put all the given values in this formula, we get:

Therefore, the vapor pressure of B is 20 kPa.