Chromatography: Quiz

Chromatography: Quiz

7 questions

1. Question 1EasyThe solvent front moved 9.00 cm and a spot moved 3.60 cm from the baseline. What is Rf?

   A0.400B2.50C0.400 cmD5.40
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   Answer: 0.400

   Rf = 3.60 cm ÷ 9.00 cm = 0.400. The units cancel, so Rf has no unit, and it is always less than 1.

2. Question 2EasyIn chromatography, a substance that is strongly attracted to the stationary phase:

   Amoves quickly and has a high RfBmoves slowly and has a low RfCdoes not affect the RfDalways stays on the baseline
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   Answer: moves slowly and has a low Rf

   It spends more time held by the stationary phase, so it travels a shorter distance.

3. Question 3MediumOn a silica TLC plate, which compound will have the highest Rf?

   Aethanoic acidBethanolChexaneDethanamide
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   Answer: hexane

   Silica is very polar. Hexane is non-polar, so it is held least and moves furthest. The others have polar O–H or N–H groups.

4. Question 4EasyWhy is the baseline drawn in pencil rather than ink?

   Apencil is easier to seeBink contains dyes that would also separateCpencil absorbs the solventDink stops the solvent moving
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   Answer: ink contains dyes that would also separate

   Graphite is insoluble and stays put; ink dyes would move and add extra spots.

5. Question 5EasyIn gas chromatography, what identifies a substance?

   Athe peak height aloneBthe retention timeCthe colour of the peakDthe total run time
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   Answer: the retention time

   Under fixed conditions each substance has its own retention time; peak area (not identity) tells how much is present.

6. Question 6MediumGC peak areas: A 600, B 1500, C 900 (equal detector response). What percentage is B?

   A20.0 %B30.0 %C50.0 %D60.0 %
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   Answer: 50.0 %

   Total = 600 + 1500 + 900 = 3000; B = 1500 ÷ 3000 × 100 % = 50.0 %.

7. Question 7HardSwitching to a more polar solvent on a silica plate will usually:

   Alower all the Rf valuesBraise all the Rf valuesCleave the Rf values unchangedDmake every spot stay on the baseline
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   Answer: raise all the Rf values

   A more polar mobile phase competes better with the polar silica for the compounds, so they all move further.