Worksheet: Distillation

In this worksheet, we will practice describing the purification of solvents and other liquids by distillation, assessing hazards and common errors.

Q1:

What is the main reason for adding a stir bar to the pot still in distillation?

  • AStirring the liquid reduces the risk of bumping.
  • BStirring the liquid prevents phase separation as the temperature increases.
  • CStirring the liquid ensures the vapor composition is uniform.
  • DStirring the liquid increases the rate of boiling, making the distillation more efficient.
  • EStirring the liquid allows it to be heated more uniformly.

Q2:

How could elastic bands be used in a vacuum distillation?

  • ATo improve the vacuum seal by applying tension to the condenser joints
  • BTo improve the condensation rate by insulating the condenser joints
  • CTo secure the thermometer in the correct position near the opening of the condenser
  • DTo prevent breakage by securing the apparatus in an upright position
  • ETo prevent water leakage by securing the condenser tubes in the correct positions

Q3:

When performing a vacuum distillation, why is it often advantageous to use a pig adapter?

  • AMultiple fractions can be collected without releasing the vacuum.
  • BThe distillate can be protected from contamination if bumping takes place.
  • CSamples of the distillate can be collected without releasing the vacuum.
  • DMultiple fractions can be collected and stored at different vacuum pressures.
  • EA larger volume of distillate can be collected.

Q4:

Which of the following precautions is most important for ensuring that the apparatus for a vacuum distillation is well sealed?

  • AThe ground glass joints are secured with Keck clips.
  • BThe condenser is maintained at a low temperature.
  • CThe receiving flask is supported by a jack.
  • DThe ground glass joints are greased.
  • EElastic bands are stretched between the still head and condenser.

Q5:

Which of the following pieces of apparatus is not typically used in distillations?

  • AHot plate
  • BKeck clip
  • CFunnel
  • DCondenser
  • EThermometer

Q6:

Which of the following pieces of apparatus is not used in distillations?

  • A
  • B
  • C
  • D
  • E

Q7:

A student sets up a vacuum distillation using the glassware shown, in addition to other apparatus. Some connectors and other parts of the glassware are labeled.

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To which position should the vacuum pump be connected?

Which positions should be supported by a clamp?

  • A6 and 10
  • B4 and 6
  • C4 and 5
  • D3 and 10
  • E3 and 5

To which position should the pig adapter be attached?

To which female connectors would the attachment of Keck clips be potentially hazardous?

  • A4 and 5
  • B6 only
  • C5 only
  • D4 and 9
  • E6 and 9

Q8:

A student sets up a distillation as shown to extract fractions from a mixture of biological oils.

In what way has the apparatus been incorrectly assembled?

  • AThe sample material is suspended in a solvent.
  • BThe apparatus is not connected to a vacuum line.
  • CThe apparatus is not connected to a nitrogen supply.
  • DThe flask containing the water is not directly heated.
  • EThe flask containing the sample material is not directly heated.

What is the main purpose of the flask containing water?

  • ATo generate steam for indirect heating and volatilization of the oil mixture
  • BTo dissolve compounds evaporating from the oil mixture
  • CTo denature enzymes in the oil mixture, which would otherwise degrade the extracted compounds
  • DTo provide a humid atmosphere around the oil mixture, increasing the rate of heat conduction
  • ETo extract water-soluble compounds from the oil mixture

Why is this procedure used in preference to a simple distillation method?

  • AWater-soluble fractions are distilled selectively.
  • BOverheating of the distillate is avoided.
  • CExposure to water lowers the boiling point of the distillate, so a higher distillation rate can be achieved.
  • DHeating with steam is more energy-efficient, so a higher distillation rate can be achieved.
  • EWater-immiscible fractions are distilled selectively.

What is the main disadvantage of this procedure, as compared to a simple distillation method?

  • AOnly water-soluble compounds can be distilled.
  • BWater is introduced into the distillate and must be removed.
  • CWater reacts with the distillate, lowering the maximum yield.
  • DGreater energy and equipment requirements lead to higher costs.
  • EThe use of water lowers the maximum achievable distillation rate.

Q9:

A student performs a distillation using the apparatus shown.

In what way has the apparatus been incorrectly assembled?

  • AThe receiving flask is not connected to the receiver adapter via a ground-glass joint.
  • BThe thermometer is positioned too high.
  • CThe crude liquid should be heated in a round-bottom flask.
  • DThe water is flowing through the condenser in the wrong direction.
  • EThe condenser should not be supported by a clamp.

What name is given to the small round objects added to the crude liquid to prevent flash boiling?

  • AStir bar
  • BAnti-bumping granules
  • CCatalyst
  • DDesiccating agent
  • EBoiling stick

A student replaces the receiver adapter with the version shown below, firmly attaching it to the receiving flask. Why is this change potentially hazardous?

  • AThere is no route for dissolved gases to escape from the distillate.
  • BThere is no route for pressure to be released from the system.
  • CThe distillate may be sucked back into the condenser.
  • DIt is more difficult to remove the receiving flask from the adapter.
  • EFirmly connecting the condenser to two flasks increases the risk of breakage.

What is the main function of the thermometer in this procedure?

  • ATo indicate when distillation of each fraction is complete.
  • BTo allow the water flow in the condenser to be set correctly.
  • CTo allow the temperature of the heating mantle to be set correctly.
  • DTo indicate the identity of each collected fraction.
  • ETo indicate the purity of each collected fraction.

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