Experiment 5: Simple Distillation and Gas Chromatography

Boiling chips are added to a solution before distillation in order to:

aid in smooth boiling

The raising of the oven temperature in GC will _____ the retention times of two compounds and _____ their separation.

shorten, decrease

You have to decide between simple distillation and other methods for the separation of two liquid compounds. For which one of the following cases would you choose simple distillation?

one compound is a nonvolatile impurity dissolved in the other liquid

Which of the following statements are true concerning azeotropes?

An unknown liquid that boils at a constant temperature could be an azeotrope. A mixture of three compounds can form an azeotrope. Azeotropes cannot be separated by simple distillation. Azeotropes can occur between substances that have similar properties (ethanol and water) and also substances that have different properties (toluene and water). Azeotropes not only form between two liquids, but can also form between mixtures of three and four compounds. Azeotropes cannot be separated by simple distillation because the vapor has the same proportions as the unboiled mixture. As a result, an unknown liquid that boils at a constant temperature is not necessarily pure and can potentially be classified as an azeotrope.

Which of the following statements is true?

As the temperature of a liquid increases, the vapor pressure increases.

Which is going to be a cause of making the retention time longer in a GC analysis?

Concentration of stationary phase liquid increased

Rapid heating can drive heated vapor up the column, preventing the formation of an equilibrium that allows the lower boiling component to separate. As a result, this will increase the number of theoretical plates and increase the separation.

False

The smaller the boiling point difference, the easier it is to separate two liquids and fewer theoretical plates are required.

False

Which statement is NOT correct about simple distillation and gas chromatography?

Neither can be used to separate organic compounds that have the same boiling point.

A diligent Chem 237 student noticed that the gas chromatograms from most of the fractions he obtained from the simple distillation of a 1:1 mixture of ethanol (bp, 78 C) and 2‐propanol (bp 82.4 C) showed two peaks. What can you say about separation of this mixture?

Simple distillation cannot be expected to separate ethanol from 2‐propanol & Gas chromatography could be used to separate a small amount of the ethanol/2‐ propanol mixture.

Which of the following statements about the collection of a GC trace is true? Select all that apply.

The amount of sample detected is proportional to the peak area. The stationary phase is a very high-boiling liquid adsorbed onto fire brick.

What error is introduced if the bulb of the thermometer, or the thermocouple, is too high in the distillation apparatus?

The recorded boiling point will be lower than actual because the bulb is not exposed to the hot vapor.

A mixture of two miscible volatile liquids is distilled. Which of the following statements are true concerning the boiling point of the distillate?

The temperature range should start being recorded from when the liquid first starts to boil. If the range is within 2 °C, then the distillate can be considered pure. The temperature range should be recorded until right before the dramatic drop in temperature.

Each vaporization-condensation is called a theoretical plate.

True

Increasing the length of the column and packing the column with glass beads or rings are methods that produce more theoretical plates.

True

The rate of heating affects the number of theoretical plates in the column.

True

Which of the following statements describe why moisture must be excluded from any sample injected into the G.C. Select all answers that apply.

Water contains dissolved salts that can degrade the column. Water is very slow to elute from the column. Water signals are very broad and make interpretation of other peaks difficult.

In order to get a VERY pure fraction of Fraction II, when should you start to collect the second fraction according to the following GC?

When the second peak begins to start

The technique of simple distillation:

can be used as a means of separation, can be used as a means of purification, is based upon differences in boiling points of two substances

When a simple distillation is carried out on a liquid containing a nonvolatile impurity, the distillation head temperature at the top of the column is:

equal to the boiling point of the pure liquid

There is a maximum quantity of sample that should be injected for preparative gas chromatography separations. This maximum quantity is determined by:

loss of resolution (separation) with increasing injection size.