The vapour pressure of two pure liquids A & B at toC are 75 & 21mm Hg resp. The two when mixed form an ideal solution. What is the mole ratio in which the two be mixed to get a solution whose V.P at t oC is 39mm Hg ?

The vapour pressure of two pure liquids A & B at toC are 75 & 21mm Hg

1.	The vapour pressure of two pure liquids A & B at toC are 75 & 21mm Hg resp. The two when mixed form an ideal solution. What is the mole ratio in which the two be mixed to get a solution whose V.P at t oC is 39mm Hg ?
Answer» The mole fraction of A is X A =0.75The mole fraction of B is X B =0.25When pressure is greater than 500 MM Hg, only liquid phase exist.The expression for the pressure at BUBBLE POINT is P bubble point =X A P Ao +X B P Bo =0.75×400+0.25×800=500 mmThe mole fractions in vapour phase are y A =0.75,y B =0.25At DEW point, P T 1 = P Ao y A + P Bo y B P T 1 = 4000.75 + 8000.25 = 8001.75 Hence, P T = 1.75800 =457.14 mm HgBelow dew point only vapour phase exists.Therefore, the reading of pressure Gauge at which only vapour phase exists is 457.14 mm of Hg