How many grams of a nonvolatile compound B (molar mass = 97.8 g/mol) would need to be added to 250 g of water to produce a solution with a vapor of 23.756 torr? The vapor pressure of water at this temperature is 42.362 torr. Please round the answer to a whole number.

Answer 1

Psoln = XH2O*PoH2O
23.756 = X*42.362
Solve for X = mole fraction H2O needed. That's approx 0.6
Then X = mole fraction B must be 1 - 0.6 = approx 0.4
mols H2O = g/molar mass = 250/18 = about 14 (approximately)
XB = mols B/(mols B + mols H2O)
0.4 = mols B/(mols B + 14). Solve for mols B.
Then mols B = grams B/molar mass B. You know mols B and molar mass B, solve for grams B. Remember the numbers I've posted are approximate only. Post your work if you get stuck.