Organic Chemistry, Separation & Analysis

11 methods · 21 questions

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Rf is the distance a spot has moved from the baseline divided by the distance the solvent front has moved from the baseline, $R_{f} = \frac{distance moved by spot}{distance moved by solvent front}$ ; the denominator is the FRONT, never the length of the paper or a fixed scale.

Trigger: any chromatogram, a table of Rf values, or the words 'solvent front'.

Instances:
(i) read the front height first, then divide each spot height by it;
(ii) if the front sits at 10 grid units, each unit is $R_{f} = 0.1$ , so counting squares gives Rf directly;
(iii) a spot on the baseline has $R_{f} = 0$ and one at the front has $R_{f} = 1$ , so every Rf lies between 0 and 1.

Linked questions (3)

NSAA 2016 Section 1, Part C (Chemistry questions) · Q43

A chromatogram was produced for 4 separate dyes (W, X, Y and Z) using filter paper and a water solvent.

A second chromatogram was produced using a mixture of two of the dyes, again using filter paper and a water solvent:

Which of the following statements, if any, are correct?

1 The concentration of dye W must be twice the concentration of dye Z.

2 The mobile phase is the filter paper.

3 The mixture in the second chromatogram contained dyes W and Y.

Two paper chromatograms drawn side by side as tall rectangles. In the left one, four spots labelled W, X, Y and Z were placed on a baseline and the solvent front is 10 cm above the baseline. The spots moved 8.2 cm (W), 3.2 cm (X), 6.4 cm (Y) and 4.1 cm (Z) up the paper, each distance marked with a double-headed arrow. In the right one, a single sample labelled mix was placed on the baseline, the solvent front is 5 cm above the baseline, and two spots appear at 3.2 cm and 4.1 cm above the baseline. The paper states that the diagram is not to scale.

Anone of them
B1 only
C2 only
D3 only
E1 and 2 only
F1 and 3 only
G2 and 3 only
H1, 2 and 3

Worked solution

Answer:D

In the first chromatogram the solvent front is 10 cm from the baseline, so the

R_{f}

values are W:

\frac{8.2}{10} = 0.82

, X:

\frac{3.2}{10} = 0.32

, Y:

\frac{6.4}{10} = 0.64

and Z:

\frac{4.1}{10} = 0.41

.

In the second chromatogram the solvent front is 5 cm from the baseline and the two spots are at 3.2 cm and 4.1 cm, giving

R_{f}

values of

\frac{3.2}{5} = 0.64

and

\frac{4.1}{5} = 0.82

. These match Y and W, so statement 3 is correct.

Statement 1 is incorrect: the distance a spot travels depends on the

R_{f}

value of the dye, not on its concentration, so nothing can be deduced about the relative concentrations.

Statement 2 is incorrect: in paper chromatography the mobile phase is the solvent (water); the filter paper is the stationary phase.

The answer is D.

NSAA 2017 Section 1, Part C (Chemistry questions) · Q41

Study the chromatogram below showing the spots obtained, labelled (i) to (v), from two sweets and pure samples of the food additives, labelled Q, R and S.

Which of the following statements about the chromatogram is/are correct?

1 Both sweet 1 and 2 contain additives R and S.

2 The

R_{f}

value for spot (iv) is half that for spot (iii).

3 The

R_{f}

value for spot (v) is 0.7.

A paper chromatogram drawn as a tall rectangle on a square grid. A horizontal line near the bottom is labelled baseline and a horizontal line near the top is labelled solvent front, ten grid divisions above the baseline. Five starting positions on the baseline are marked with crosses and labelled, from left to right, sweet 1, sweet 2, Q, R and S. Sweet 1 shows two spots, labelled (i) one division above the baseline and (ii) four divisions above it. Sweet 2 shows three spots, labelled (iii) two divisions up, (iv) four divisions up and (v) seven divisions up. Q shows a single spot six divisions up, R a single spot one division up and S a single spot four divisions up.

Anone of them
B1 only
C2 only
D3 only
E1 and 2 only
F1 and 3 only
G2 and 3 only
H1, 2 and 3

Worked solution

Answer:D

Read the chromatogram against its grid. The solvent front lies 10 grid divisions above the baseline, so each division corresponds to an

R_{f}

of 0.1. The spots sit at the following heights: (i) 1 division, (ii) 4 divisions, (iii) 2 divisions, (iv) 4 divisions, (v) 7 divisions; the pure additives sit at Q 6 divisions, R 1 division and S 4 divisions.

Statement 1: sweet 1 gives spots (i) and (ii), which line up with R (1 division) and S (4 divisions), so sweet 1 does contain R and S. Sweet 2 gives spots (iii), (iv) and (v): (iv) matches S, but neither (iii) at 2 divisions nor (v) at 7 divisions matches R at 1 division. Sweet 2 does not contain R, so statement 1 is incorrect.

Statement 2:

R_{f} = \frac{distance moved by spot}{distance moved by solvent}

. Spot (iv) has

R_{f} = 4/10 = 0.4

and spot (iii) has

R_{f} = 2/10 = 0.2

. The value for (iv) is double, not half, that for (iii), so statement 2 is incorrect.

Statement 3: spot (v) has

R_{f} = 7/10 = 0.7

, so statement 3 is correct.

Only statement 3 is correct.

The answer is D.

NSAA 2018 Section 1, Part C (Chemistry questions) · Q49

Paper chromatography was used to separate three mixtures of amino acids. The mixtures were labelled P, Q and R.

Each mixture contains some of the five amino acids in the following table. The

R_{f}

values were measured for each amino acid with the solvent used to produce the chromatogram.

amino acid asparagine glutamic acid glycine leucine valine R_{f} value 0.50 0.30 0.26 0.71 0.61

Which of the following statements is/are correct?

1 Mixture P contains valine and glycine.

2 Leucine is found in all three mixtures.

3 Glutamic acid is the least mobile amino acid with this solvent.

4 Mixtures P and Q both contain asparagine.

Outline of a rectangular chromatography paper. A dashed horizontal line near the top is labelled solvent front and a dashed horizontal line near the bottom is labelled start line; three small crosses on the start line are labelled P, Q and R from left to right. Short unlabelled tick marks run up both vertical edges. Above P are three spots, at roughly 0.6 and 0.5 of the way from the start line to the solvent front and a lower one at roughly 0.26. Above Q are three spots at roughly 0.7, 0.6 and 0.5. Above R are two spots at roughly 0.7 and 0.6 and a pair of touching spots at roughly 0.3 and 0.26.

A1 and 2 only
B1 and 4 only
C2 and 3 only
D3 only
E4 only

Worked solution

Answer:B

The

R_{f}

value of a spot is its distance from the start line divided by the distance moved by the solvent front. Measuring each spot on the chromatogram as a fraction of the start line to solvent front distance and matching it to the table identifies the amino acids.

Mixture P has spots at about

0.61

0.50

and

0.26

: valine, asparagine and glycine. Mixture Q has spots at about

0.71

0.61

and

0.50

: leucine, valine and asparagine. Mixture R has spots at about

0.71

and

0.61

plus two spots close together near

0.30

and

0.26

: leucine, valine, glutamic acid and glycine.

Statement 1 is correct: P contains valine and glycine. Statement 2 is wrong because leucine is absent from P. Statement 3 is wrong: the least mobile amino acid is the one with the smallest

R_{f}

, which is glycine (

0.26

), not glutamic acid (

0.30

). Statement 4 is correct: P and Q both contain asparagine.

Statements 1 and 4 only are correct.

The answer is B.