chromatography.

Question 1

What is Chromatography ?

Answer 1

Chromatography is an important analytical technique because it allows chemists to separate substances in complex mixtures. There are a variety of types of chromatography, which can be used in different contexts.

Question 2

What is Qualitative analysis?

Answer 2

Qualitative analysis means any method of identifying the chemicals in a sample.

Quantitative analysis means working out how much of a particular chemical there is in a sample.

Any analysis must be carried out on a sample of a substance that is representative of the bulk of the substance. This means that the results from the analysis can be used to draw conclusions about the rest of the substance.

It is important to mix solutions thoroughly before testing, and to take many samples at random from the solution when taking repeat readings.

It is important to follow standard procedures when analysing chemicals. This ensures that the results are reliable and valid.

Question 3

What is Quantitative analysis?

Answer 3

means working out how much of a particular chemical there is in a sample.

Question 4

WHy is it important to follow standard procedures when analysing chemicals.

Answer 4

This ensures that the results are reliable and valid

Question 5

Any analysis must be carried out on a sample of a substance that is representative of the …. of the substance.

Answer 5

Any analysis must be carried out on a sample of a substance that is representative of the bulk of the substance. This means that the results from the analysis can be used to draw conclusions about the rest of the substance.

It is important to mix solutions thoroughly before testing, and to take many samples at random from the solution when taking repeat readings.

.

Question 6

This means that the results from the analysis can be used to draw conclusions about the rest of the substance.

It is important to mix solutions thoroughly before testing, and to take many samples at random from the solution when taking repeat readings.

.
What else should you remember when sampling?

Answer 6

It is important to mix solutions thoroughly before testing, and to take many samples at random from the solution when taking repeat readings.

.

Question 7

Standard procedures cover what 3 things?

Answer 7

Standard procedures cover the following:

collecting samples

storing samples

preparing samples for analysis

Question 8

In chromatography, substances are s…………… as they travel in a ……………phase which passes through a …………. phase. Different substances travel at ………… speeds, so some move ……………. than others in a given time

Answer 8

In chromatography, substances are separated as they travel in a mobile phase which passes through a stationary phase. Different substances travel at different speeds, so some move further than others in a given time

Question 9

In paper chromatography, the stationary phase is what? .

Answer 9

In paper chromatography, the stationary phase is paper. The mobile phase may either be an aqueous (water-based) liquid or a non-aqueous organic (carbon-based) solvent. An example of an organic solvent is propanone - which is the main chemical in nail varnish remover.

Question 10

The mobile phase may either be an …………………. Give an example

Answer 10

The mobile phase may either be an aqueous (water-based) liquid or a non-aqueous organic (carbon-based) solvent. An example of an organic solvent is propanone - which is the main chemical in nail varnish remover.

Question 11

For each chemical in the sample, there is a dynamic equilibrium between the……………..phase and the…………phase. The overall separation depends upon how strongly………….the chemicals are to the mobile and the stationary phases.

Answer 11

For each chemical in the sample, there is a dynamic equilibrium between the stationary phase and the mobile phase. The overall separation depends upon how strongly attracted the chemicals are to the mobile and the stationary phases.

Question 12

What is Thin layer chromatography (TLC)?

Answer 12

Thin layer chromatography (TLC) is similar to paper chromatography but instead of paper, the stationary phase is a thin layer of an inert substance (eg silica) supported on a flat, unreactive surface (eg a glass plate).

Question 13

What are some advantages of TLC over paper chromatography. Give an example.

Answer 13

TLC has some advantages over paper chromatography. For example:

the mobile phase moves more quickly through the stationary phase

the mobile phase moves more evenly through the stationary phase

there is a range of absorbencies for the stationary phase

Question 14

What does TLC produce that is more useful than paper chromatography,

Answer 14

TLC tends to produce more useful chromatograms than paper chromatography, which show greater separation of the components in the mixture - and are therefore easier to analyse.

Question 15

A chromatogram can be compared with a chromatogram of a ………… ……………material to identify which …………….the mixture contained.

Answer 15

A chromatogram can be compared with a chromatogram of a standard reference material to identify which chemicals the mixture contained.

Question 16

Sometimes the substances being separated are colourless. In this case, what can be used to show where the spots are?

Locating agents bind to the chemicals in the spots. Sometimes, another chemical is then added, which reacts with the locating agent to produce a coloured spot, or the chromatogram is put under ultraviolet light and the locating agent glows to show where the spots are.

Answer 16

Locating agents

Question 17

What do Locating agents do?

Answer 17

Locating agents bind to the chemicals in the spots. Sometimes, another chemical is then added, which reacts with the locating agent to produce a coloured spot, or the chromatogram is put under ultraviolet light and the locating agent glows to show where the spots are.

Question 18

What is Rf value?

Answer 18

Rf value

The movement of a substance during chromatography, relative to the movement of the solvent, is measured by calculating its retardation factor (Rf).

Question 19

Rf value = distance travelled by substance/distance travelled by solvent

What is the formula for Rf value?

Answer 19

Rf value = distance travelled by substance/distance travelled by solvent

Calculating the Rf value allows chemists to identify unknown substances because it can be compared with Rf values of known substances under the same conditions.

Question 20

Calculating the Rf value allows chemists to identify what?

Answer 20

Calculating the Rf value allows chemists to identify unknown substances because it can be compared with Rf values of known substances under the same conditions.

Question 21

What is Gas chromatography?

Answer 21

In gas chromatography (GC), the mobile phase is an inert gas (eg helium).

The stationary phase is a very thin layer of an inert liquid on an inert solid support - such as beads of silica packed into a long thin tube (this flexible tube is coiled many times inside a thermostatically-controlled oven to keep it at a constant temperature).

Question 22

What is GC used for?

Answer 22

GC is used to separate complex mixtures. It is much better at this than thin-layer or paper chromatography. This is because it is more sensitive - allowing the determination not only of what chemicals are in the mixture, but also how much of each chemical there is.

Question 23

What does a gas chromatogram show ?

Answer 23

A gas chromatogram might show the time along the x axis and the strength of response along the y axis.

Question 24

What is retention time?

Answer 24

The amount of time that a substance takes to pass through the column is called it retention time. The retention time of an unknown substance can be compared with standard reference data to help to identify it.

Question 25

What three main pieces of information can be gathered from a gas chromatogram:

Answer 25

Three main pieces of information can be gathered from a gas chromatogram:

the number of compounds in the mixture - represented by the number of peaks

how much of each compound is present - represented by the height of the peak (higher = more)

the retention time - indicated by the position of the peak