Question

At 55.0 ?C, what is the vapor pressure of a solution prepared by dissolving 78.4 g of LiF in 265 g of water? The vapor pressure of water at 55.0 ?C is 118 mmHg. Assume complete dissociation of the solute.

Answer 1

We have, P_solution = x_solvent x P_solvent

Where P is the pressure and x is the mole fraction

Now, here, P_water = 118 mmHg

Moles of water = (265 g)/(18.016 g/mol) = 14.70 mol

Moles of LiF = (78.4 g)/(25.94 g/mol) = 3.022 mol

Therefore, mole fraction of water = 14.70 /[14.70 +(2 x 3.022)] = 0.7086

(Note: LiF dissociate in to two ions. Therefore, while finding the mole fraction of water, we will have to multiply the number of moles of LiF by 2)

P_solution = 0.7086 x 118 mmHg = 83.61 mmHg