AP Chemistry Chromatography

Calculating Rf, reading a chromatogram, and explaining the order of spots with intermolecular forces, in four worked examples.

Chromatography questions mix a small calculation with an explanation. You measure distances to get an Rf value, then use that value to say which substance is more polar and why, in terms of attractions to the stationary and mobile phases. The Unit 3 review states the idea in two lines; this page works through the numbers and the wording, using the intermolecular forces that decide where each spot ends up.

How Chromatography Separates a Mixture and What Rf Measures

A mixture is placed on a stationary phase (paper or a silica-coated plate) and a mobile phase (the solvent) moves past it. Each component is attracted to both phases, and the balance between the two sets how fast it travels:

Stronger attraction to the stationary phase means the substance lags behind and ends low on the plate.
Stronger attraction to the mobile phase means it is carried along and ends high on the plate.

The retention factor puts a number on that: R_f = (distance moved by the spot) ÷ (distance moved by the solvent front), both measured from the origin line. In the common classroom setup the stationary phase is polar and the solvent is less polar, so a more polar substance sticks to the plate and has a smaller R_f.

Worked Example 1: Calculating Rf Values From Measured Distances

After a chromatogram is developed, the solvent front is 8.0 cm above the origin line. Spots A, B, and C are at 2.0 cm, 4.8 cm, and 6.4 cm above it, measured to the center of each spot. Find each R_f.

A: 2.0 ÷ 8.0 = 0.25.
B: 4.8 ÷ 8.0 = 0.60.
C: 6.4 ÷ 8.0 = 0.80.

The units cancel, so R_f has none, and it cannot exceed 1 because no spot travels farther than the solvent. Measure both distances from the origin line, not from the bottom edge of the plate, and report the right number of significant figures; see significant figures for the rules.

Worked Example 2: Ranking Polarity and Explaining It With Intermolecular Forces

The plate in Example 1 has a polar silica stationary phase and a nonpolar solvent. Rank A, B, and C by polarity and explain the order.

Ranking: lowest R_f is most polar, so A (most polar) > B > C (least polar).
Explanation: A has the strongest attraction to the polar stationary phase, through dipole-dipole attractions or hydrogen bonding, and is the least attracted to the nonpolar solvent, so it is held back and moves the least. C has weaker attractions to the plate and is better matched with the nonpolar solvent, whose attractions are mostly London dispersion forces, so it is carried farthest.

A full-credit answer names both phases, names the type of attraction, and links it to the distance traveled. Why molecules are polar or nonpolar is explained in molecular polarity, and the nonpolar attractions are covered under London dispersion forces.

Worked Example 3: Identifying the Components of an Unknown Ink

On one plate, an unknown ink shows spots 3.0 cm and 6.0 cm above the origin line, and the solvent front is 7.5 cm. Reference dyes run on the same plate have R_f values of red 0.40, yellow 0.80, and blue 0.10. Which dyes are in the ink?

Spot 1: 3.0 ÷ 7.5 = 0.40, which matches red.
Spot 2: 6.0 ÷ 7.5 = 0.80, which matches yellow.
Conclusion: the ink contains red and yellow, and no blue because nothing appears near 0.10.

Matching works only when the references and the unknown share the same stationary phase, solvent, and temperature, which is easiest when they are on the same plate. A different solvent changes every R_f, so values from separate experiments cannot be compared directly.

Worked Example 4: Reading Retention Times From a Column

Two compounds, P and Q, are passed through a column with a polar stationary phase and a nonpolar mobile phase. P leaves the column after 2.1 minutes and Q after 6.4 minutes. Which is more polar, and what would a more polar solvent do?

More polar compound: Q. It spends more time attracted to the polar stationary phase, so it takes longer to exit.
More polar solvent: it competes with the stationary phase for the polar compounds and carries them along, so both retention times decrease, and the compounds spend more of their time in the mobile phase.

Column chromatography reports retention time instead of R_f, but the reasoning is identical: a longer time means a stronger attraction to the stationary phase. Distillation, the other separation technique in the course, rests on a related idea: differences in vapor pressure reflect differences in the strength of the attractions between particles.

Five Chromatography Practice Questions With Answers

A spot moves 3.6 cm while the solvent front moves 9.0 cm. What is its R_f?
Show answer
3.6 ÷ 9.0 = 0.40.
Can an R_f value be greater than 1?
Show answer
No. A spot cannot travel farther than the solvent front, so the ratio is at most 1.
A compound has R_f = 0.30 in one solvent and 0.70 in another. Why do the values differ?
Show answer
The mobile phase changed. A solvent that competes better with the stationary phase, or that dissolves the compound better, carries it farther, so R_f depends on the conditions.
On a polar plate with a nonpolar solvent, compound X has R_f = 0.15 and compound Y has R_f = 0.70. Which is more polar?
Show answer
X. It is held more strongly by the polar plate, so it moves less.
In a column with a polar stationary phase, compound R elutes at 2.1 min and compound S at 6.4 min. Which has the stronger attraction to the stationary phase?
Show answer
S. A longer retention time means it is held more strongly by the stationary phase.

Common Chromatography Mistakes

Measuring from the bottom edge of the plate instead of the origin line.
Measuring to the edge of a spot instead of its center, or forgetting to mark the solvent front before it dries.
Reading a higher R_f as more polar. With a polar plate and a nonpolar solvent it is the opposite.
Comparing R_f values from different solvents or plates. The conditions must match.
Explaining separation by size or density. The cause is attraction to each phase, which comes from intermolecular forces.
Placing the origin line below the solvent level, which washes the sample into the container.

Lab-style questions like these are a reliable source of free-response points in Unit 3. To see how a stronger Unit 3 would shift your overall estimate, the AP Chem Score Calculator turns section scores into a 1–5 range.

Frequently Asked Questions

What is the Rf value in chromatography?

The retention factor, Rf, is the distance a spot travels divided by the distance the solvent front travels, both measured from the origin line. It has no units and always falls between 0 and 1.

What does a higher Rf mean?

A higher Rf means the substance spent more of its time in the mobile phase and less attracted to the stationary phase. With a polar stationary phase and a nonpolar solvent, a higher Rf means a less polar substance.

Why is the origin line drawn above the solvent level?

If the origin line were below the solvent level, the sample would dissolve into the solvent in the container instead of traveling up the plate, and no separation would occur.

Does column chromatography use Rf values?

No. Column chromatography is usually compared by retention time, the time a substance takes to pass through the column. A stronger attraction to the stationary phase gives a longer retention time.

Can the polarity trend reverse?

Yes. In reversed-phase chromatography the stationary phase is nonpolar and the mobile phase is polar, so more polar substances move farther. Always identify which phase is polar before reading the trend.

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