posted by Hannah

The vapor pressure of chloroform is 173.11 mm Hg at 25 degrees celsius. A nonvolatile, nonelectrolyte that dissolves in chloroform is aspirin. Calculate the vapor pressure of the solution at 25 degrees celsius when 6.243g of aspirin (180.1g/mol) are dissolved in 244.5g of chloroform.

Chloroform=119.40

I did 6.243g aspirin / 180.1g = 0.0346 moles

244.5g / 119.40g = 2.04 moles

Is this correct so far? I do not know how to convert solvent to mole fraction. Thank you!

1. DrBob222

So far so good.
Xaspirin = moles aspirin/total moles.
XCHCH3 = moles CHCl3/total moles.

2. Hannah

So I would do 0.0346/180.1 for aspirin and 2.04/119.40 for CHCH3?

3. DrBob222

No.
You have mole aspirin already as 0.0346(but I would go back and recalculate because you are allowed more s.f. than that).
You have mole CHCl3 already as 2.04 (but I would recalculate and use as many s.f. as allowed)--You aren't looking at the next number before writing the answer from your calculator.
XCHCl3 = moles CHCl3/total moles. Total moles is not molar mass it is total moles. 2.05 + 0.0347 = ? (or whatever those new numbers are).
Actually, if you are determining the vapor pressure of CHCl3 you don't need Xaspirin, only XCHCl3.

4. Hannah

Yes it just says calculate the vp of the solution when aspirin is dissolved in the CHCl3. So then I do not have to calculate the moles/total moles?

5. DrBob222

Yes you do. You can't calculate XCHCl3 without total moles and you can't get total moles without getting moles aspirin. You don't need to calculate mole fraction aspirin (that's Xaspirin) but you do need to calculate mole. mole and mole fraction are not the same. I think you are confusing the two.

6. Hannah

Yes im sorry I am sort of confused. So is the total number of moles what they gave me in the problem, 6.243?

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