Question

1) A gas mixture containing 75% air, 20% of A and 5% of B (by volume) is in equilibrium
with an aqueous solution of A and B at a temperature such that the saturation pressure
of pure A is 3 bar, and that of pure B is 8 bar. Estimate the composition of the liquid if
the total pressure is 2 bar. State any assumptions you have made.

Answer 1

Assumption: We have to assume that the gas phase is ideal and the liquid phase is an ideal solution. So we will see that Dalton's law and Raoult's law applies.

Assumption: Assume Air is insoluble in the aqueous solution.

Raoult's law is given as,

Where pa is the partial pressure of the component in gas.

xa is it's mole fraction of that component in liquid.

pas is the vapour pressure of the pure component/saturation pressure.

Now using Dalton's law we see that,

ya is the mole fraction of that component in vapour.

P is the total pressure of the system.

So for component A, we will do the calculation as,

So,

Now,

vapour pressure of the pure component/saturation pressure of A,

For component B, we will do the calculation as,

So,

Now,

vapour pressure of the pure component/saturation pressure of B,

So the composition of liquid is given as, 13.33% component A , 1.25% component B and 85.42% water.