MCAT General Chemistry Practice Test 4: Phases

Home > MCAT Test > MCAT general chemistry practice tests

Test Information

Question 5 questions

Time minutes

See All test questions

Take more free MCAT general chemistry practice tests available from maintests.com.

Lyophilization, or freeze drying, is a technique used to remove water from samples. Lyophilization uses sublimation to convert frozen, solid water directly to water vapor. This technique offers advantages over liquid-solvent removal techniques in that it can be performed at low temperatures, resulting in minimal damage to heat-sensitive samples.

A chamber containing the sample solution is attached to the lyophilizer, which freezes the sample at a temperature less than 0°C and then applies a vacuum, generally holding a pressure less than 0.006 atm. Under these conditions the frozen water sublimes and water vapor is pulled from the sample chamber into a condenser held at -50°C, where it is refrozen and held immobile.

The phase diagram for water is shown below, with its unique negatively sloped equilibrium line between the solid and liquid phase. At low pressures the solid phase of water is favored as long as any solute in the water is reasonably dilute. However, the freezing point of water decreases as the concentration of solute is increased. If concentrated enough, the sample can melt instead of sublime when pressure is decreased. In addition, volatile solvents often cannot be removed from the chamber by the condenser as their freezing point at low pressures is below -50°C. Volatile solvents remaining in the gaseous environment can compound the impurity of the remaining water, favoring the liquid phase even more.


Figure 1 Phase diagram of water

1. The addition of which of the following substances is most likely to result in the melting of a sample during lyophilization?

  • A. (CH3)2CHOH
  • B. CH3CH2COOH
  • C. CH3CH2OCH2CH3
  • D. CH3(CH2)4OH

2. If the temperature inside the chamber is -60°C, what pressure must the chamber be at in order for the sublimation reaction to be at equilibrium?

  • A. 3 × 10-4 atm
  • B. 3 × 10-3 atm
  • C. 1 atm
  • D. 125 atm

3. Lyophilization would most likely be employed industrially in which of the following separations?

  • A. Salicylic acid dissolved in methanol
  • B. An enzyme in aqueous solution
  • C. NaCl in water
  • D. A small molecular drug dissolved in dichloromethane

4. Sublimation of water inside the sample chamber causes the temperature of the chamber to:

  • A. increase.
  • B. decrease.
  • C. stay the same.
  • D. change in a manner dependent on the identity of the solvent.

5. The heat of sublimation of water is 46 kJ/mol. If heat is transferred to the sample by the environment at a rate of 0.1 kJ/min, approximately how long will it take to lyophilize 40 cm3 of frozen water (density = 0.91 g/mL) ?

  • A. 7.7 hours
  • B. 15.3 hours
  • C. 77.0 hours
  • D. 153.0 hours