[vmdenslow]

Would I do this type of equation by using Raoult's Law? The actual problem says "How many grams of sucrose C12H22O11 must be added to 55 g of water to give a solution with a vapor pressure 2.0 mmHg less than that of pure water at 20 degrees Celcius (the vapor pressure of water at 20 degrees Celcius is 17.5 mmHg).

I know that you need to convert the grams to moles, but I don't remember how to change the grams of sucrose to moles without knowing either how many grams or moles there were to begin with.

[Donaldson Tan]

using raoult's law means assume ideal solution.

Using 1atm, 20C as the reference state,

µ_i = µ^* + RT.ln(X_i)

µ_i = µ^* + RT.ln(P_i/P^*)

comparing the 2 equivalent equations, you end up with

X_i = P_i/P^*

let molar mass of sucrose be M
let mass of sucrose used be m
=> X_i = 55/(55 + m/M)

..to give a solution with a vapor pressure 2.0 mmHg less than that..

=> P_i/P^* = (17.5 - 2)/17.5

from here, you should've no problem arriving at the answer