Chromatography

Question 1

chromatography is used to separate ___ substances

Answer 1

coloured

Question 2

absorption refers to a substance in the mobile ____ phase binding to the surface of the stationary ____ phase

Answer 2

gas/liquid; liquid/solid

Question 3

the solid phase should bind ______ to the substances to be separated

Answer 3

reversibly

Question 4

why should the solid phase bind reversibly to the substances to be separated?

Answer 4

so the compound can actually separate and move

Question 5

the surface area of a solid should be

Answer 5

large

Question 6

what are the advantages of chromatography? (4)

Answer 6

1. highly selective
2. simple apparatus
3. can be used for small-medium samples (mg-10xg)
4. can be used for large samples (100xg)

Question 7

why is it important to shut the cover of the developing chamber?

Answer 7

1. the chamber must be saturated with solvent vapour

2. if the vapours escape, it will mess up the Rf

Question 8

the level of liquid in the developing chamber should be ___ the applied spots of the sample

Answer 8

beneath

Question 9

why must the solvent not be allowed to travel to the top of the chromatography plate?

Answer 9

the solvent front is used as reference in calculating Rf

Question 10

how can 2 plates be developed at once?

Answer 10

back to back in the chamber

Question 11

what happens if a solvent is too polar in TLC?

Answer 11

all components move to the top of the plate

Question 12

If using a polar solid phase, a small Rf value means the substance is (more/less polar)

Answer 12

more polar (attracted to the polar solid)

Question 13

list the 7 requirements for an absorbant/ solid phase

Answer 13

1. large surface area
2. polar functional groups
3. stable/inert to maintain nature over time
4. react reversibly with components
5. preferably white
6. cheap, readily available
7. highly selective

Question 14

why is a pencil not used to mark the chromatograph?

Answer 14

ink may run and interfere

Question 15

why should air bubbles not be made in the absorbant of the column?

Answer 15

they restrict flow by limiting contact with sand, this increases band size

Question 16

why must elution in the column start with the least polar component?

Answer 16

because they will travel farthest and fastest. Banding will be affected

Question 17

would you use TLC to separate 5ml of sample?

Answer 17

no, too much, use column or partition

Question 18

why must a pure sample be spotted on the same plate as an unknown?

Answer 18

to compare colour and Rf in order to identify unknown

Question 19

what 3 important characteristics of the alumina in the column that are used for chromatography?

Answer 19

1. white
2. highly selective
3. large surface area

Question 20

state the 5 differences between TLC and column

Answer 20

1. TLC faster
2. TLC = spots, column = bands
3. TLC = simpler, cheaper
4. TLC = cant separate large volume
5. TLC goes against gravity, column goes with gravity

Question 21

how is Rf calculated?

Answer 21

distance from center of spot from origin / distance of solvent front from origin