How a Chromatographic Separation Works · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.
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Why mixtures split into bands

Science · Chemistry · ages 19-20
Name ______________________   Date ____________

  1. Cs and Cm are equilibrium concentrations in the stationary and mobile phases. Which expression defines distribution constant K?

    • Cs + Cm
    • Cs / Cm
    • Cm / Cs

  2. Which pair describes the phases used in chromatography?

    • Two flowing phases that react with each other
    • A stationary phase and a flowing mobile phase
    • A stationary phase and an empty vacuum chamber

  3. An unretained marker is used to measure the hold-up or dead time tM.

    Circle one:   True   False

  4. In a linear partition model, k = 4 and Vs/Vm = 0.2. Calculate distribution constant K.

    Answer: ______________

  5. With constant mobile-phase composition, temperature and flow, tM is 2 minutes and tR is 8 minutes. Calculate retention factor k.

    Answer: ______________

  6. Two analytes have k values of 1 and 4 on the same column under the same conditions. In the linear partition model, which has the larger K?

    • The analyte with k = 4
    • The analyte with k = 1
    • Both have equal K values

  7. For peaks with the same retention time, larger plate number N corresponds to a narrower peak. The plates are hypothetical equilibrium stages.

    Circle one:   True   False

  8. A less polar stationary phase lowers an analyte's measured K from 10 to 2. Linear partitioning applies and Vs/Vm stays fixed. What happens to k?

    • It becomes five times as large
    • It remains unchanged by the swap
    • It becomes one fifth as large
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Why mixtures split into bands W1-mt_uKOPELqWzG-s1

  1. Cs / Cm · K compares stationary-phase concentration with mobile-phase concentration: Cs/Cm.
  2. A stationary phase and a flowing mobile phase · The stationary phase stays in place. The mobile phase flows and carries the sample.
  3. True · The marker provides the reference transit time without stationary-phase retention.
  4. 20 · K = k/(Vs/Vm) = 4/0.2 = 20.
  5. 3 · k = (8 - 2)/2 = 6/2 = 3. It has no units.
  6. The analyte with k = 4 · Both analytes share Vs/Vm, so the larger k corresponds to the larger K.
  7. True · Plate number describes efficiency. A theoretical plate is not a physical disc or a device that squeezes a band.
  8. It becomes one fifth as large · K becomes 2/10, or one fifth, of its original value. At fixed Vs/Vm, k changes by the same factor.
Worksheet · LightMySky