The vapour pressure of two liquids P and Q are 80 and 60 torr, respectively. The total vapour pressure of solution obtained by mixing 3 moles of P and 2 moles of Q would be :

According to Raoult's law, the total vapor pressure of a solution is the sum of the partial vapor pressures of each component. The partial vapor pressure of each component is given by the product of its mole fraction and its vapor pressure as a pure substance. For a solution of 3 moles of P and 2 moles of Q, the mole fractions are 3/5 for P and 2/5 for Q. The partial vapor pressure of P is (3/5) 80 torr = 48 torr, and the partial vapor pressure of Q is (2/5) 60 torr = 24 torr. The total vapor pressure of the solution is the sum of these partial vapor pressures: 48 torr + 24 torr = 72 torr.