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Chromatography principles and applications: NEET MCQs with answers

90 questions on this topic of Purification and Characterisation of Organic Compounds (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. Given below are two statements: Statement I: Gas chromatography is primarily used for the separation and analysis of non-volatile and thermally unstable organic compounds. Statement II: In gas-liquid chromatography, the mobile phase is an inert gas such as helium or nitrogen. In the light of the above statements, choose the correct answer:

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
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Answer: (C) Statement I is incorrect but Statement II is correct

Statement I is incorrect because gas chromatography requires samples to be volatile without decomposition; thermally unstable or non-volatile compounds cannot be analyzed directly. Statement II is correct because an inert gas like helium, argon, or nitrogen serves as the mobile phase carrier gas.

medium

2. In a TLC experiment on silica gel, a chemist spots three compounds: P (hexane-1-ol), Q (hexanoic acid), and R (hexane). If the plate is developed using a weakly polar solvent, which compound will have the smallest Rf value?

  1. Compound R
  2. Compound Q
  3. Compound P
  4. Both Compound P and Compound R will have the same smallest Rf
Show answer and explanation

Answer: (B) Compound Q

Silica gel is a polar stationary phase. More polar and acidic compounds interact most strongly with silica gel via hydrogen bonding and dipole-dipole forces. Hexanoic acid (-COOH) is more polar and forms stronger hydrogen bonds than hexane-1-ol (-OH) and the non-polar hexane. Therefore, hexanoic acid is held most firmly on the plate and has the lowest migration rate, giving the smallest Rf value.

hard

3. A student spotted a sample 1.5 cm from the bottom edge of a 10.0 cm tall TLC plate. The plate was placed in a chamber filled with eluent up to a depth of 0.8 cm. When the solvent front reached 1.5 cm from the top edge, the spot of compound Z was located 4.2 cm from the bottom edge of the plate. What is the Rf value of compound Z?

  1. 0.42
  2. 0.38
  3. 0.32
  4. 0.49
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Answer: (B) 0.38

The baseline is at 1.5 cm from the bottom. The top of the plate is at 10.0 cm. Solvent front distance from bottom = 10.0 - 1.5 = 8.5 cm. Distance travelled by solvent front from baseline = 8.5 - 1.5 = 7.0 cm. The spot is at 4.2 cm from the bottom, so distance travelled by compound Z from baseline = 4.2 - 1.5 = 2.7 cm. Rf = 2.7 / 7.0 ≈ 0.386 ≈ 0.38.

medium

4. Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A): The retardation factor (Rf) value of an organic compound on a TLC plate is completely independent of the solvent system used. Reason (R): Retardation factor (Rf) depends only on the chemical structure of the solute and the nature of the stationary adsorbent. In the light of the above statements, choose the correct answer from the options given below:

  1. Both (A) and (R) are true and (R) is the correct explanation of (A)
  2. Both (A) and (R) are true but (R) is not the correct explanation of (A)
  3. (A) is true but (R) is false
  4. Both (A) and (R) are false
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Answer: (D) Both (A) and (R) are false

The Rf value strongly depends on the mobile phase (eluent) because varying the polarity of the mobile phase changes the partitioning/adsorption equilibrium of the solute. Hence, Rf is not independent of the solvent system, making both Assertion (A) and Reason (R) false.

easy

5. What is the maximum possible theoretical value for the retardation factor (Rf) of any solute in thin layer chromatography?

  1. 0.0
  2. 1.0
  3. 0.5
  4. Infinity
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Answer: (B) 1.0

The retardation factor (Rf) is defined as the distance moved by the substance divided by the distance moved by the solvent front. Since a solute cannot move farther than the advancing solvent front, Rf can never exceed 1.0.

hard

6. In a gas-liquid chromatography (GLC) separation of volatile organic esters, the components are separated on a column packed with an inert solid coated with a high-boiling non-volatile liquid. The separation is fundamentally governed by:

  1. Differential adsorption of the esters on the solid support surface.
  2. Differences in vapor pressures and differential partition coefficients between carrier gas and liquid stationary phase.
  3. Differences in the rates of chemical reaction with the carrier gas.
  4. Differential precipitation of esters inside the column.
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Answer: (B) Differences in vapor pressures and differential partition coefficients between carrier gas and liquid stationary phase.

Gas-liquid chromatography is a partition technique where separation is governed by differences in compound volatilities (vapor pressures) and their differential partition coefficients (solubilities) between the gaseous mobile phase and the liquid stationary phase film.

hard

7. Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A): During thin-layer chromatography (TLC), the spotted TLC plate must never be immersed in the eluent up to or above the sample application baseline. Reason (R): If the solvent level is higher than the baseline, the spotted sample will dissolve directly into the solvent reservoir instead of ascending the plate. In the light of the above statements, choose the correct answer from the options given below:

  1. Both (A) and (R) are true and (R) is the correct explanation of (A)
  2. Both (A) and (R) are true but (R) is not the correct explanation of (A)
  3. (A) is true but (R) is false
  4. (A) is false but (R) is true
Show answer and explanation

Answer: (A) Both (A) and (R) are true and (R) is the correct explanation of (A)

The baseline spot must remain strictly above the initial solvent level in the development chamber. If the solvent level reaches or covers the spot, the compounds dissolve into the bulk mobile phase reservoir at the bottom of the beaker, ruining the chromatogram.

medium

8. On a TLC plate, the baseline is drawn 1.5 cm from the bottom edge. After development, the solvent front reaches 9.5 cm from the bottom edge. A spot of compound P travels to a position 5.5 cm from the bottom edge. What is the retardation factor (Rf) of compound P?

  1. 0.50
  2. 0.58
  3. 0.42
  4. 0.69
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Answer: (A) 0.50

Distance moved by solvent front from baseline = 9.5 cm - 1.5 cm = 8.0 cm. Distance moved by compound P from baseline = 5.5 cm - 1.5 cm = 4.0 cm. Therefore, Rf = 4.0 / 8.0 = 0.50.

easy

9. On a silica TLC plate developed with a moderately polar solvent, three compounds X, Y, and Z show Rf values of 0.15, 0.48, and 0.82 respectively. If the same mixture is subjected to adsorption column chromatography with the same stationary phase and solvent, what is the order of elution?

  1. X first, then Y, then Z
  2. Y first, then X, then Z
  3. Z first, then Y, then X
  4. Z and X elute together, followed by Y
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Answer: (C) Z first, then Y, then X

A higher Rf value in normal-phase TLC indicates that the solute interacts weakly with the stationary phase and moves faster. Therefore, in column chromatography on the same adsorbent, Z (Rf = 0.82) is held least tightly and elutes first, followed by Y (Rf = 0.48), and X (Rf = 0.15) which is adsorbed most strongly elutes last.

hard

10. Consider the elution of three compounds: cyclohexanol, cyclohexanone, and cyclohexane through a standard silica gel column using dichloromethane as the eluent. Which of the following represents the correct sequence in which these compounds emerge from the bottom of the column?

  1. Cyclohexane, then cyclohexanol, then cyclohexanone
  2. Cyclohexanol, then cyclohexanone, then cyclohexane
  3. Cyclohexane, then cyclohexanone, then cyclohexanol
  4. Cyclohexanone, then cyclohexane, then cyclohexanol
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Answer: (C) Cyclohexane, then cyclohexanone, then cyclohexanol

On silica gel, adsorption affinity increases with increasing polarity of functional groups: alkane (non-polar) < ketone (moderately polar) < alcohol (strongly polar, forms H-bonds with silanol groups). The least strongly adsorbed compound elutes first. Hence, cyclohexane emerges first, followed by cyclohexanone, and finally cyclohexanol.

medium

11. The retardation factor (Rf) in thin layer chromatography is:

  1. Time taken by substance / time taken by solvent
  2. Mass of substance / mass of adsorbent
  3. Distance moved by the substance / distance moved by the solvent front
  4. Distance moved by solvent / distance moved by substance
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Answer: (C) Distance moved by the substance / distance moved by the solvent front

Rf = distance travelled by the substance from the baseline divided by the distance travelled by the solvent front.

NCERT Class 11

hard

12. Two compounds, A and B, have Rf values of 0.35 and 0.70, respectively, on a silica gel TLC plate developed with a mixture of hexane and ethyl acetate (4:1). If the same mixture is resolved using a pure hexane mobile phase, what changes in their Rf values are expected?

  1. Both Rf values will increase.
  2. Rf(A) will increase while Rf(B) will decrease.
  3. Both Rf values will decrease, but Rf(B) remains greater than Rf(A).
  4. Rf values will remain unchanged because the stationary phase is identical.
Show answer and explanation

Answer: (C) Both Rf values will decrease, but Rf(B) remains greater than Rf(A).

Pure hexane is less polar than a hexane-ethyl acetate mixture. A less polar mobile phase has lower eluting power on a polar adsorbent like silica gel, causing both compounds to interact longer with the stationary phase and thus reducing both Rf values; however, B remains less polar than A and retains a higher Rf.

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