2. The vapor pressure of liquid octane, C8H18, is 100. mm Hg at 339 K. A sample

Question

2. The vapor pressure of liquid octane, C8H18, is 100. mm Hg at 339 K. A sample of C8H18 is placed in a closed, evacuated 518 mL container at a temperature of 339 K. It is found that all of the C8H18 is in the vapor phase and that the pressure is 61.0 mm Hg. If the volume of the container is reduced to 373 mL at constant temperature, which of the following statements are correct? Choose all that apply.

a. No condensation will occur.

b. Some of the vapor initially present will condense.

c. The pressure in the container will be 100. mm Hg.

d. Only octane vapor will be present.

e. Liquid octane will be present.

2.b The vapor pressure of liquid acetone, CH3COCH3, is 40.0 mm Hg at 264 K. A sample of CH3COCH3 is placed in a closed, evacuated container of constant volume at a temperature of 402 K. It is found that all of the CH3COCH3 is in the vapor phase and that the pressure is 50.0 mm Hg. If the temperature in the container is reduced to 264 K, which of the following statements are correct? Choose all that apply.

a. pressure in the container will be 40.0 mm Hg.

b. No condensation will occur.

c. Liquid acetone will be present.

d. Some of the vapor initially present will condense.

e. Only acetone vapor will be present.

Explanation / Answer

Moles of C8H18, n= PV/RT = (61/760)* (518/1000)/ (0.0821*339)=0.001494 moles

Even the volume is reduced, the no of moles will not change,

Hence pressure in the second case at constant temperature is P= nRT/V= 0.001494*0.0821*339/0.373=0.11 atm = 0.11*760 = 84 mm Hg

Since partial pressure is less than vapor pressure, there is no condensation. Only octane vapor will be present.

2.

2. let the volume be V, number of moles of acetone = PV/RT= (50/760)*V/(0.0821*402)=0.01993V

When the temperature is reduced to 264K without change in volume, V

Pressure = nRT/V= 0.01993V*0.0821*264/V = 0.043 atm = 32.83 mm Hg. Since this is less than the vapor pressure of acetone, there will not be any condensation. Pressure of container remains at 40 mm Hg. Only acetone vapor will be present.

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