Sem1 Unit8 Phase Equilibria

SK017 Unit 8

Past Year Examination Questions

Unit 8: Phase Equilibria Jan 99 1.

a) State four colligative properties. b) Calculate the mass of glucose, C6H12O6 needed to decrease the freezing point of 150 g water by 0.75°C. [Kf = 1.86°C kg mol−1] <10.887> c) The vapour pressure of heptane and octane at 20°C are 473 Pa and 140 Pa respectively. Calculate the vapour pressure of a solution containing 100 g heptane and 57 g octane. <363>

Jun 99 2.

a) State the Raoult’s law. At 30°C, the vapour pressure of pure liquids P and Q are 0.15 atm and 0.05 atm respectively. When equilibrium was reached between the vapour state and their liquid solutions, calculate the mole fraction of vapour P if the mole fraction of P in the liquid is 0.6? <0.82> b) The freezing point and the molal freezing point depression constant Kf for benzene are 5.5 °C and 5.1°C kg mol−1 respectively. When 3.6 g of an organic compound, R was dissolved in 60.0 g of benzene, the freezing point changed to 4.5°C. Calculate the molar mass of R. <306>

Jan 00 3.

Carbon dioxide has a triple point at temperature −57°C and pressure 5 atm while its critical point is at temperature 31°C and pressure 73 atm. Sketch a labelled phase diagram for carbon dioxide and explain how the melting point of carbon dioxide changes with increasing pressure.

Jul 00 4.

a) What is the meaning of the colligative properties of solution? b) Explain a law which correlate the change in vapour pressure of a solvent with the concentration of solute present. c) The table below shows data obtained at 25°C for a miscible system: 1-propanol and water. Mole fraction of 1-propanol, x Vapour pressure of water, P (mmHg)

0

0.02

0.05

0.10

0.20

0.40

0.60

0.80

0.95

1.00

23.7

23.5

23.2

22.7

21.8

21.7

19.9

13.4

4.2

0

Plot a graph of water vapour pressure versus the mole fraction of 1-propanol. State whether the graph obtained obeys the law stated in (b). Explain.

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SK017 Unit 8

Past Year Examination Questions

5.

The table below shows the boiling points for pure trichloromethane, ethanol and azeotropic mixture which is formed from the mixture of trichloromethane and ethanol. Boiling point/°C 61.2 78.5 53.6

Liquid Trichlomethane Ethanol Azeotropic mixture

a) What is meant by the azeotropic mixture? b) Sketch and label the diagram of vapour pressure versus composition of the trichloromethane-ethanol system. c) Sketch and label the diagram of temperature versus composition of the trichloromethaneethanol system. d) Based on the diagram in (c), explain how you can obtain pure CHCl3 liquid. Aug 01 6.

The boiling point of H2O is 100°C while the boiling point of HNO3 is 86°C. When these two solutions were added, the vapour pressure of the mixture was less than the vapour pressure of pure H2O and pure HNO3. The azeotropic mixture was formed when the per cent composition of HNO3 is 68%. a) Give the meaning of azeotropic mixture. b) State the type of deviation exhibited by the mixture. Explain. c) Sketch and label the diagrams of temperature-composition and vapour pressurecomposition for the mixture. d) State the product and residue for the distillation of a mixture containing 50% H2O and 50% HNO3.

Aug 02 7.

A solution was formed by adding carbon tetrachloride to ethanol. The interacting forces between molecules ethanol and between carbon tetrachloride molecules were found to be greater than the interacting forces between ethanol and carbon tetrachloride molecules. i. The solution deviates from Raoult’s law. Explain. ii. There was a change in temperature while forming the solution. Explain. iii. State whether the value of ∆H is positive or negative?

8.

The following diagram represents the vapour pressure for substance Y. Pressure

(mmHg)

u

760 p

q

r

s

t

v temperature Explain the points u and v. State the phases that exist at points q and r. 9.

Calculate the mass of urea, H2NCONH2 needed to add to 1.0 kg of water so that the solution formed will boil at 100.25°C. Hence explain why the boiling point of urea in water is higher compared to the boiling point of pure water. 38

SK017 Unit 8

Past Year Examination Questions

[boiling point of water =100°C; Kb water=0.51°C mol−1 kg]

<29.41g>

Sept 03 10.

An amount of 11.40 g non-volatile substance T was dissolved in 600 g of water. The freezing point of the solution is -0.50°C. Calculate the relative molecular mass of T and the boiling point of its solution. <71, 100.136°C> −1 −1 [Kf water =1.86 °C m , Kb water =0.51 °C m ]

11.

(a)

Define Raoult’s law. A mixture of acetone-water has a negative deviation from Raoult’s law with P°water< P° acetone

Sketch a graph of vapour pressure versus mole fraction of the mixture. The mixture of acetone-water exhibits a negative deviation. Explain this statement in terms of the attractive forces and the vapour pressures of the solution. (7M) (b)

What is an azeotropic mixture? X and Y form an azeotropic mixture with composition of 42% X. The mixture boils at a temperature of 341 K and pressure of 760 torr. At this pressure, X and Y boil at 351 K and 353 K respectively. Sketch the phase diagram of temperature versus per cent composition for the mixture. Do these mixtures show positive or negative deviation from ideality? Explain. (8M)

State the distillate collected and the residue left in the flask if a mixture of 60% X and 40% Y is fractionally distillated. Oct 04 12.

The temperature-composition phase diagram of mixtures of liquids A and B is shown in the following figure. 160

Temperature /°C

155 150 145 140 135 130 125 120

i.

ii. 13.

0.2 0.4 0.6 0.8 1.0 xA When a mixture of A and B is distilled in a simple distillation apparatus, what can you say about the composition of the distillate and the composition of the residue? Explain. When an equimolar mixture of A and B is distilled in a simple distillation apparatus, what would the boiling point be? Estimate the composition of the distillate.

Table below shows data phase equilibrium diagram for carbon dioxide. Pressure (atm) Temperature (°C) Triple point 5.2 −57 Critical point 73 31 a) Sketch a phase diagram for carbon dioxide. b) By referring to the phase diagram, explain why solid carbon dioxide at ordinary temperature and pressure does not melt, instead it sublimes.

39

SK017 Unit 8

Past Year Examination Questions

c) Explain the phase changes that could possibly occur when a sample of carbon dioxide in a closed vessel under 1 atm pressure and a temperature of -78°C is pressurised isothermally to 10 atm, followed by isobaric heating to 20°C. Oct 05 14.

When 0.833 g of vitamin B2 is dissolved in 18.1 g of water, the freezing point of this solution is −0.227°C. [The molal freezing point depression constant of water is 1.86°C kg mol−1] i. Calculate the molality of the solution. <0.122> ii. Determine the molar mass of vitamin B2. <377.2>

15.

(a) What is an azeotropic mixture? (b) The boiling point of an azeotropic mixture containing B and C with a mole fraction xB = 0.7 is 120°C. Pure B and C boil at 65°C and 90°C respectively. Sketch a labelled phase diagram from the above data. (c) State any deviation from Raoult’s law. (d) Show in the diagram above the composition variation during successive boilings and condensations of a mixture with initial mole fraction, xC=0.8. (e) A mixture with initial mole fraction, xB=0.5 is distilled using fractional distillation. Name the distillate and residue.

Oct 06 16.

State Raoult’s law. Verify whether each of the observations in table below obey Roult,s law. Explain

. Observations 1

The vapour pressure of a liquid solvent F was 0.8 atm. A nonvolatile component G was added at a mol fraction of 0.25. The vapour pressure of the mixture dropped to 0.6 atm.

Observations 2

A solution was made by dissolving 6 g of a nonvolatile substance of molecular weight 60 g/mol in 450 g of toluene, C6H5CH3.. The solution showed a vapour pressure of 38 mmHg at 26ºC whereas the pure toluene has a vapour pressure of 40 mmHg at the same temperature.

Oct 07 17.

(a) TABLE 2 shows phase equilibrium data for substance D.

40

SK017 Unit 8

Past Year Examination Questions

TABLE 2 Sublimation point Triple point Critical point Melting point 1 Melting point 2 Boiling point 1 Boiling point 2

Pressure (atm) 2.5 5.0 40.0 15.0 35.0 20.0 30.0

Temperature (oC) - 80 - 60 30 - 30 - 15 10 25

Plot and label a phase diagram for D. Does D sublime at 20 atm? Explain your answer.

(b) Ethanol, C2H5OH and methanol, CH3OH form an ideal solution. The vapour pressures of pure ethanol and pure methanol are 6.5 kPa and 10.3 kPa, respectively at 25 oC. Calculate the vapour pressure of a solution made from 30 g of ethanol and 20 g of methanol at 25 oC. [C2H5OH = 46; CH3OH = 32] Oct 08 18.

(a) A solution containing 0.5 g of an unknown non-volatile solute is added to 100 mL of water. This solution is separated from a volume of pure water by a semi-permeable membrane. When the system reaches equilibrium, the difference in height, h, between the solution and the solvent is 5.6 cm. Assuming that the density, ρ, of the solution is 1.0 g cm-3, calculate the molecular mass of the unknown solute. (Given: Pressure = ρgh, gravitational acceleration, g = 9.8 m s-2) (b) A mixture consisting of 0.40 mol fraction of hexane, C6H14, and 0.60 mol fraction of heptane, C7H16, forms an ideal solution. State two properties of an ideal solution and explain why a mixture of heptane-hexane forms an ideal solution. By drawing an appropriate phase diagram, state how the mixture of these liquids can be separated. [boiling point: C6H14 = 69oC, C7H16 = 98oC]

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