[jerm174]

Acetone and ethyl acetate are organic liquids often used as solvents. at 30°C, the vapour pressure of acetone is 285 mm Hg and the vapour pressure of ethyl acetate is 118 mm Hg. What is the vapour pressure in mm Hg at 30°C of a solution prepared by dissolving 25.0 g of each liquid?

Why is it that when I use the formula P_soln=P_solvX_solv I get the wrong answer? What is the difference between this and the formula P_total=P^°_AX_A+P^°_BX_B, which gives the correct answer? Isn't the vapour pressure of the solution the same as the vapour pressure total?

(The answer given is 219 mm Hg)

[Borek]

First equation assumes solvent pressure to be lower because dissolved substances are nonvolatile. Here both liquids are volatile, so the assumption doesn't hold.

[jerm174]

Thanks again, Borek !