An aqueous solution which contains 42 % by weight ( w / W ) of a volatile liquid " A " of molar mass of 140 ~g / mol , has a vapour pressure of 2141.4 mm of Hg at 37^ C . What is the vapour pressure of the pure liquid "A"? Vapour pressure of water at 37^ C is 2019.1 mm .

Step-by-step solution

Let the total mass of the solution be 100 g . Mass of solute A = 42 g . Mass of solvent (water) = 100 - 42 = 58 g . Molar mass of A ( M_A ) = 140 g/mol . Molar mass of water ( M_W ) = 18 g/mol . Number of moles of A ( n_A ) = 42 140 = 0.3 mol . Number of moles of water ( n_W ) = 58 18 = 3.222 mol . Mole fraction of A ( x_A ) = n_A n_A + n_W = 0.3 0.3 + 3.222 = 0.3 3.522 0.08518 . Mole fraction of water ( x_W ) = 1 - x_A = 1 - 0.08518 = 0.91482 . According to Raoult's Law for a solution of two volatile liquids: P_ t