Question

Determine the vapour pressure lowering at 25 degree Celsius of 1.25molality sucrose solution assume the solution behaves ideally (sucrose is C12H22O11). The vapour pressure of pure water at 25 degrees Celsius is 23.8torr ( hint 50.0g of sucrose was used).

Answer 1

delta P = X_soluteP^o_solvent
So you must determine both X for the solute and mols for the solvent. First you must determine the grams H2O used in the experiment.

molality = 1.25 = mols sucrose/kg H2O
Solve for kg H2O. Convert to grams H2O then to mols H2O using mols = grams/molar mass.
mols sucrose = 50g/molar mass sucrose.
Total mols = mols sucrose + mols H2O
Xsucrose = mols sucrose/total mols.
Plug Xsucrose and Po into the equation at the top and solve for delta P.
Post your work if you get stuck.