An unknown nonelectrolyte and nonvolatile compound is dissolved in enough pure water to make 229 ml of solution. The vapour pressure above the solution drops from 233.7 torr to 229.1 torr when 82.3g of the compound is dissolved at constant temperature. Determine the molar mass of the unknown compound. Assume no change in volume occurs upon dissolution.

Question

DrBob222 · Accepted Answer

mols solute = 82.3 g/molar mass = ? Don't know molar mass.
delta P = X_solute*P⁰_solvent.
(233.7 - 229.1) = Xsolute*233.7
Xsolute =
Xsolute = 0.0197 = mols solute/(mols solute+moles solvent) where
mols solvent = 229 g H2O/18.02 = 12.71 and let n = moles solute. Then
n/(n+12.71) = 0.0197 and n of solute = 0.255
Then grams solute = mols solute/molar mass solute. You have grams and mols, solve for molar mass. Post your work if you get stuck.

Carly · Answer

Can you show steps how u got n of solute = 0.255 I'm not sure how to make n by itself in the equation n/(n+12.71)=0.0197

DrBob222 · Answer

This is math;not chemistry but i'm happy to show you.
n/(n+12.71)=0.0197
n = 0.0197 x (n + 12.71)
n = 0.0197n + 0.2504
1n - 0.0197n = 0.2504
1.0000n - 0.0197n = 0.2504
0.9803n = 0.2504
n = 0.2504/0.9803 = 0.255 :-)