Partition coefficients
Cambridge A-Level Chemistry (9701) · Unit 25: Equilibria · 7 flashcards
Partition coefficients is topic 25.2 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 25 — Equilibria , alongside Acids and bases. In one line: The partition coefficient (Kpc) is the ratio of the concentration of a solute in two immiscible solvents at equilibrium. It indicates how a solute distributes itself between the two solvents. Kpc = [solute in solvent 1] / [solute in solvent 2].
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 7 flashcards — 2 definitions, 3 key concepts and 2 calculations — covering the precise wording mark schemes reward. Use the 2 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
The term 'partition coefficient' (Kpc)
The partition coefficient (Kpc) is the ratio of the concentration of a solute in two immiscible solvents at equilibrium. It indicates how a solute distributes itself between the two solvents. Kpc = [solute in solvent 1] / [solute in solvent 2]
What the Cambridge 9701 syllabus says
2025-2027 syllabus, in our words · A2 LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- State the meaning of partition coefficient, Kpc.
- Calculate a partition coefficient and use it, where the solute's physical state is the same in both solvents.
- Understand how the polarity of the solute and of each solvent affects the size of a partition coefficient's numerical value.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9701 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Define the term 'partition coefficient' (Kpc).
The partition coefficient (Kpc) is the ratio of the concentration of a solute in two immiscible solvents at equilibrium. It indicates how a solute distributes itself between the two solvents. Kpc = [solute in solvent 1] / [solute in solvent 2]
Write the expression for the partition coefficient (Kpc) of compound X between water and ether.
Kpc = [X in water] / [X in ether], with both concentrations measured at equilibrium. By convention, the solvent named first in 'between water and ether' goes in the numerator. Always check which way round a question defines Kpc.
A compound Y has a Kpc of 4 between water and chloroform, with chloroform in the numerator. If 10g of Y is dissolved in 100cm³ of water and shaken with 100cm³ of chloroform, what mass of Y will be in the chloroform layer?
Let 'x' be the mass in chloroform. Then (10-x)/0.1 is the concentration in water, and x/0.1 is the concentration in chloroform. Kpc = (x/0.1)/((10-x)/0.1) = 4. Solving for x gives x=8g.
Explain how the polarities of the solute and solvents affect the partition coefficient.
The 'like dissolves like' principle applies. A polar solute will have a higher Kpc value in a polar solvent, while a non-polar solute will have a higher Kpc value in a non-polar solvent, due to stronger intermolecular forces of attraction.
What effect does increasing the polarity of solvent 1 have on the partition coefficient (Kpc) if a polar solute is used?
If solvent 1 becomes more polar, then the solute will likely dissolve more favourably in solvent 1. This will lead to a larger [solute in solvent 1] and therefore a larger Kpc.
State two assumptions that are made when calculating partition coefficients.
1. The solute is in the same physical state (e.g. not associated or dissociated) in both solvents. 2. The two solvents are immiscible/do not mix.
Suggest how you can improve the extraction of a compound from an aqueous solution into an organic solvent using the partition coefficient principle.
Perform multiple extractions using smaller volumes of the organic solvent. This ensures a larger overall proportion of the solute transfers to the organic phase than with a single extraction using the entire volume.
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Partition coefficients sits alongside these A-Level Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Partition coefficients deck
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