Chemistry
posted by Katrina on .
A solution is prepared by mixing 0.0400 mol CH2Cl2 and 0.0700 mol of CH2Br2 at 25°C. Assuming the solution is ideal, calculate the composition of the vapor (in terms of mole fractions) at 25°C. At 25°C, the vapor pressures of pure CH2Cl2 and pure CH2Br2 are 133 and 11.4 torr, respectively.

XCH2Cl2 = 0.04/total moles.
XCH2Br2 = 0.07/total moles.
partial pressure CH2Cl2 = XCH2Cl2*133 = ??
partial pressure CH2Br2 = XCH2Br2*11.4
Total pressure = PCH2Cl2 + PCH2Br2 = ??
XCH2Cl2 in vapor = (partial pressure CH2Cl2/total P of vapor)
XCH2Br2 in vapor = (partial pressure CH2Br2/total P of vapor). 
I did those steps and got .869 for CH2Cl2 and .130 CH2Br2 (assuming that both is in the correct number of sigfigs), but they don't seem to be right. I just want to know what I am doing wrong, or is this also the answer you got.

I obtained those numbers also. If you are keying your answer into a computer based "answer sheet" I suspect the problem is in the significant figures. Note that 0.04 and 0.07 have only 1 s.f. each. I would round the 0.870 value to 0.9 and the 0.130 value to 0.1