Posted by Sara on .
A solution is prepared by mixing 0.0400 mol CH2Cl2 and 0.0800 mol CH2Br2 at 25 degrees C. Assume solution is ideal, calculate the composition of the vapor (in terms of mole fractions) at 25 C. At 25 C, vaporpressure of pure CH2Cl2 and pure CH2Br2 are 133 and 11.4 torr.

chemistry 
DrBob222,
Sum the moles to get total moles.
mole fraction CH2Cl2 = #moles/total moles.
mole fraction CH2Br2 = #moles/total moles.
vapor pressure CH2Cl2 = mole fraction CH2Cl2 x normal vapor pressure CH2Cl2.
vapor pressure CH2Br2 = mole fraction Ch2Br2 x normal vapor pressure CH2Br2.
Total pressure is the sum of the partial pressures of each component. 
chemistry 
Sara,
Thanks. My answer is still coming up wrong, thought it was sig figs, but I'm getting 43.9 and 7.52.......mole fractions are .33 and .66. What gives???

chemistry 
DrBob222,
I didn't round like you did AND you short changed the s.f. You are allowed three from 0.0800 and 0.400 but you rounded to 2. I leave those extra 3s and 6s in the calculator and make the other calculations.
So I divide 0.04/0.12 = 0.3333333 and multiply on the spot by 133 to obtain 44.33 torr.
0.08/0.12 = 0.666666 and multiply on the spot by 11.4 = 7.60 (note that by rounding to 0.66 the number is 7.52 and rounding to 0.67 the number is 7.63a big difference with just one digit). So I would add 44.33 and 7.60 = 51.93. You are allowed three s.f. and I would round that final number to 51.9 torr. Check my work.