SOLUTIONS [ HW-1 ]
1. Calculate the molality of a sulphuric acid solution in which the mole fraction of
water is 0.85
2. A 6.90 M solution of KOH in water contains 30% by mass of KOH. Calculate the
density of the KOH solution. (Molar mass of KOH = 56 g mol)
3. Determine the molarity of an antifreeze solution containing 250 g water mixed
with 222 g ethylene glycol. The density of the solution is 1.07 g//m
4. At what partial pressure oxygen will have a solubility of 0.05 gram per litre in
water at 293 kelvin. Henry's constant KH in H2O at 293 kelvin is 34.86. Assume
the density of the solution is the same as that of the solvent.
5. The Henry's law constant for oxygen dissolved in water is 4.34 X104 atm at 25°C.
If the partial pressure of oxygen in air is 0.2 atm under atmospheric conditions,
calculate the concentration (in moles per litre) of dissolved oxygen in water in
equilibrium with air at 25°C
6. What concentration of nitrogen should be present in a glass of water at room
temperature? Assume a temperature of 25°C, a total pressure of 1 atmosphere and
a mole fraction of nitrogen in the air as 0.78 [KH for nitrogen = 8.42 X10-7 M/mm
Hg]
7. Benzene and toluene form a nearly ideal solution at 313 K. The vapour pressures
of pure benzene and toluene are 160 mm of Hg and 60 mm of Hg respectively.
Calculate the partial pressure of benzene and toluene and the total pressure over
the following solutions: (1) containing equal weights of benzene and toluene, (ii)
containing 1 mole of benzene and 4 moles of toluene. (iii) containing equal
molecules of benzene and toluene.
8. The vapour pressures of benzene and toluene at 293 K are 75 mm and 22 mm Hg
respectively. 23.4 g of benzene and 64.4 g of toluene are mixed. If the two form
an ideal solution, calculate the mole fraction of benzene in the vapour phase
assuming that the vapour is in equilibrium with the liquid mixture at this
temperature
9. Vapour pressure of an aqueous solution of glucose is 750 mm of Hg at 373 K.
Calculate the molality and mole fraction of solution.
SOLUTIONS [ HW-2 ]
9. Vapour pressures of benzene and toluene in a mixture at 50°C are given in mm by
P = 179 Xb + 92 where Xb is the mole fraction of benzene.
Calculate
(a) Vapour pressures of pure benzene and toluene at 50°C.
(b) Vapour pressure of a liquid mixture obtained by mixing 224 g of benzene and 184 g
of toluene.
(c) If the vapours are removed and condensed into liquid and again brought to the
temperature of 50°C, what would be the mole fraction of benzene in the vapour phase?
11. (i) Benzoic acid completely dimerises in benzene. What will be the vapour pressure
of a solution containing 61 g of benzoic acid per 500 g benzene when the vapour pressure
of pure benzene at the temperature of the experiment is 66.6 torr?
(ii) What would have been the vapour pressure in the absence of dimerisation?
3. The degree of dissociation of Ca (NO2), in a dilute aqueous solution containing 7.0 g
of the salt per 100 g of water at 100°C is 70 percent. If the vapour pressure of water at
100°C is 760 mm, calculate the vapour pressure of the solution.
4. (a) What is van’t Hoff factor? What types of values can it have if in forming the
solution, the solute molecules undergo
(i) Dissociation? (ii) Association?
(b) How many mL of a 0.1 M HCl solution are required to react completely with 1 g of a
mixture of Na2CO3 and NaHCO3 containing equimolar amounts of both?
5. 68.4 g of sugar (molecular weight = 342) is dissolved in 1000 g of water.
What is (a) freezing point (b) boiling point (c) vapour pressure at 20°C (d) osmotic
pressure of the solution at 20°C? The density of the solution at 20°C is 1.024 g cm³. The
vapour pressure of water at 20°C is 17.633 mm. The K, and K, values for water are 1.873°
and 0.516° respectively.
6. Two elements A and B form compounds having molecular formula AB₂ and AB4.
When dissolved in 20 g of benzene (C6H6), 1 g of AB₂ lowers the freezing point by 2.3 K
whereas 1.0 g of AB, lowers it by
1.3 K. The molal depression constant for benzene is 5.1 K kg mol -1. Calculate atomic
masses of A and B.
7. The air is a mixture of a number of gases. The major components are oxygen and
nitrogen with approximate proportion of 20% is to 79% by volume at 298 K. The water is
in equilibrium with air at a pressure of 10 atm. At 298 K, if the Henry's law constants for
oxygen and nitrogen are 3.30 x 107mm and 6.51 x 107 mm respectively, calculate the
composition of these gases in water.
8. (i) What is de-icing agent? How does it function ?
(ii) 19.5 g of CH2FCOOH is dissolved in 500 g of water. The depression in freezing
point of water observed is 1.0°C. Calculate the van't Hoff factor and dissociation constant
of fluoroacetic acid.
10. A solution of glycerol (C3H8O3; molar mass = 92 g mol-1) in water was prepared
by dissolving some glycerol in 500 g of water. This solution has a boiling point of
100.42 °C. What mass of glycerol was dissolved to make this solution? Kb for
water = 0.512 K kg mol-1.
11. State Henry’s law and mention two of its important applications.
SOLUTIONS [ HW-3 ]
1. A solution of glycerol (C3H8O3; molar mass = 92 g mol-1) in water was prepared
by dissolving some glycerol in 500 g of water. This solution has a boiling point of
100.42 °C. What mass of glycerol was dissolved to make this solution? Kb for
water = 0.512 K kg mol-1.
2. (a) Differentiate between molarity and molality in a solution. What is the effect of
temperature change on molarity and molality in a solution?
(b) What would be the molar mass of a compound if 6.21 g of it dissolved in 24.0 g of
chloroform form a solution that has a boiling point of 68.04°C. The boiling point of pure
chloroform is 61.7°C and the boiling point elevation constant, Kb for chloroform is
3.63°C/m.
3. The freezing point of a solution containing 0.3 g of acetic acid in
30.0 g of benzene is lowered by 0.45°C. Calculate the van't Hoff factor (Kf for benzene =
5.12 K kg mol-1).
4. Calculate the boiling point of a solution containing 0.61 g of benzoic acid in 50 g of
carbon disulphide assuming 84% dimerisation of the acid. The boiling point and K of CS,
are 46.2°C and 2.3 K kg mole
5. The depression in freezing point of water observed for the same amount of acetic
acid, trichloroacetic acid and trifluoroacetic acid increases in the order given above.
Explain briefly.
6. (a) Define Azeotropes and explain briefly minimum boiling azeotrope by taking
suitable example.
(b) The vapour pressures of pure liquids A and B are 450 mm and 700 mm of Hg
respectively at 350 K. Calculate the composition of the liquid mixture if total vapour
pressure is 600 mm of Hg. Also find the composition of the mixture in the vapour phase.
7. (a) State the following:
(i) Henry's law about partial pressure of a gas in a mixture, (
(ii) Raoult's law in its general form in reference to solutions.
(b) What would be the molar mass of a compound if 6.21 g of it dissolved in 24.0 g of
chloroform form a solution that has a boiling point of 68.04°C. The boiling point of pure
chloroform is 61.7°C and the boiling point elevation constant K, for chloroform is
3.63°C/m.
8. To 500 cm³ of water, 3.0 x 10-3 kg of acetic acid is added. If 23% of acetic acid is
dissociated, what will be the depression in freezing point? Kf and density of water are 1.86
K kg mol-1 and 0.997 g cm³ respectively.
9. You are given one litre each of 0.5 M HCl and 0.2 M HCl. How should they be
mixed so as to give maximum volume of 0.4 M HCI?
10. Calculate the depression in the freezing point of water when 10 g of
CH₂CH₂CHCICOOH is added to 250 g of water. K = 1.4 ×
10-3, K = 1.86 K kg mol-1.
12. A 1.00 molal aqueous solution of trichloroacetic acid (CCl3COOH) is heated to
its boiling point. The solution has the boiling point of 100.18°C. Determine the
van’t Hoff factor for trichloroacetic acid. (Kb for water = 0.512 K kg mol-1)
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