How do you calculate vapor pressure of water above a solution prepared by adding 24 g of lactose (C12H22O11) to 200 g of water at 338 K? (Vapor-pressure of water at 338 K 187.5 torr.)

VP (solution) = 186.3 Torr

Explanation:

Given 24g Lactose (C₁₂H₂₂O₁₁) IN 200g H₂O @338K (65°C) & 187.5Torr

VP (soln) = VP (solvent) - [mole fraction of solute (X) ·VP (solvent) ] = > Raoult's Law

VP (H₂O) @65°C&187.5Torr = 187.5Torr

moles Lactose = (24g/342.3g/mol) = 0.0701mole Lactose

moles Water = (200g/18g/mol) = 11.11mole Water

Total moles = (11.11 + 0.0701) mole = 11.181mole

mole fraction Lac = n (lac) / [n (lac) + n (H₂O) ] = (0.0701/11.181) = 6.27 x 10⁻³

VP (solution) = 187.5Torr - (6.27 x 10⁻³) 187.5Torr = 186.3Torr