PHM1213 Physical Pharmacy
Tutorial
Colligative Properties and Distillation
1. The vapor pressure of pure liquid solvent A is 0.80 atm. When a nonvolatile
substance B is added to the solvent, its vapor pressure drops to 0.60 atm. What is
the mole fraction of component B in the solution?
2. The vapor pressure of pure water at 25°C is 23.76 torr. The vapor pressure of a
solution containing 5.40 g of a nonvolatile substance in 90.0g of water is 23.32
torr. Compute the molecular weight of the solute.
3. At 25°C, the vapor pressure of water is 23.8 torr. What is the vapor pressure of a
solution prepared by dissolving 65.0 g of C6H12O6 (a nonvolatile solute) in 150 g
of water? (Assume the solution is ideal)
4. At 25°C the vapor pressures of benzene (C6H6) and toluene (C7H8) are 93.4 and
26.9 torr, respectively. A solution made by mixing 60.0 g of benzene and 40.0 g
of toluene is prepared. At what applied pressure, in torr, will this solution boil?
5. Benzene and toluene help get good engine performance from lead-free gasoline.
At 40°C, the vapor pressure of benzene is 180 torr and that of toluene is 60 torr.
Suppose you wished to prepare a solution of these liquids that will have a total
vapor pressure of 96 torr at 40°C. What must be the mole percent concentrations
of each in the solution?
6. Ethyl acetate, a solvent for certain plastics, has a vapor pressure of 72.8 torr at
20°C and a vapor pressure of 186.2 torr at 40°C. Estimate the value of Δhvap for
this solvent in units of kilojoules per mole.
7. The vapor pressure of liquid nitrogen is 28.1 tor at -216°C and 47.0 torr at 213°C. calculate the heat of vaporization of N2 in units of kJ/mol. Estimate the
normal boiling point of liquid nitrogen.
8. The heat of vaporization of toluene (used to make trinitrotoluene, TNT) is 38.0
kJ/mol. The vapor pressure of toluene is 20.0 torr at 18.4°C. What will its vapor
pressure be at 25°C?
9. Describe how and explain why a standard solution (of precisely known
concentration) of HCl can be prepared from aqueous solution of HCl without
chemical analysis.
10. Explain on a molecular basis why benzene and toluene form nearly ideal solutions
with each other.
Prepared by: Kausar Ahmad
Date: 8/12/03
Answers to PHM1213 Tutorial
Colligative Properties and Distillation
1. P = xP° x(A) = P = 0.60 atm = 0.75
P° 0.80 atm
x(B) = 1 – x(A) = 1 – 0.75 = 0.25
2. P = P°x
X = 23.32 torr = 0.981 = 5.00
3.76 torr
5.00 + z
(5.00 + z)(0.981) = 5.00
z = 0.0968 mol
5.40 g/ 0.0968 mol = 55.8 g/mol
3. 22.8 torr
4. 69.4 torr
5. benzene 30%, toluene 70%
6. 35.8 kJ/mol
7. 4.88 kJ/mol, bp - 189°C, 84 K
8. 28.3 torr
9. The HCl- H2O eutectic has a definite composition at any given pressure.
Distillation of the mixture will sooner or later yield a constant boiling mixture, which
has a composition which has been determined previously to an accuracy sufficient for
analytical determinations (to at least four significant figures)
10. They have very similar structures, differing only by the replacement of a
hydrogen atom by a methyl group.
Prepared by: Kausar Ahmad
Date: 8/12/03