Distillation Questions

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1. One mole of a compound A, with vapor pressure 400mmHg at 50oC, mixed with 3 moles of compound B, with vapor pressure 480mmHg at 50oC to form a homogeneous solution. What is the vapor pressure of mixture at 50oC?

XA =moles of A\total No. of moles
XA =1\4 XB=3\4
=400*1\4 + 480*3\4 = 460mmHg

2. Why should a distilling flask be filled not less than 1\3 filled or more than 2\3 full?

This is to allow the liquid being heated room to expand as it turns to a vapor. Also, 1\3 would cause too much pressure, and 2\3 not enough pressure. In addition, too much in the flask, it risks bumping over. That happens when there becomes an isolated area of high heat, and there is a sudden boiling that could be too vigorous and material floods over.

3. A 50% aqueous solution of ethanol (50ml total) is distilled and collected in 10ml fractions. Predict the poiling range of each fraction.

Fractions vol. collected (ml) temp. oC(predicted)
1 10 78
2 20 78
3 30 78-100
4 40 100
5 50 100

For A compound = 78-82oC
For B compound = 100oC

4. What is the mole fraction of each component if 3.9g of benzene(C6H6) is dissolved in 4.6g of toluene (C7H8)

Mole of benzene=3.9g\(6*12+6)g\mol
= 0.05 mol
Moles of toluene=4.6g\(7*12+8)g\mol
=0.05 mol
Mole fraction of benzene=mole fraction of toluene = 0.05\(0.05+0.05) =0.5
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