Define a partition coefficient
Describe the equilibrium concentration ratio for a solute distributed between two immiscible solvents.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 25. Use the equilibrium expressions and data stated in the question; written practice is self-checked and human subject-teacher review remains pending.
Before you begin
- Use AS equilibrium expressions, mole calculations, logarithms, and Brønsted–Lowry acid–base ideas.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does a partition coefficient compare?
What you will learn
- State the meaning of partition coefficient Kpc
- Use a clearly specified solvent order in the concentration ratio
- Recognise that the solute must be the same species and physical state in both solvents
A partition coefficient compares equilibrium concentrations
When a solute distributes between two immiscible solvents, the partition coefficient at a fixed temperature is the ratio of its equilibrium concentrations in the two solvents. In this course, use Kpc = [solute]organic/[solute]aqueous unless a question explicitly defines a different order.
The ratio compares the same solute species in the same physical state in both solvents. Each concentration is measured after the system has reached distribution equilibrium.
State the convention and conditions
Kpc is dimensionless when the same concentration units are used in numerator and denominator. Its value depends on the chosen solvent pair, temperature and how well the solute interacts with each solvent.
If a question gives a named numerator/denominator convention, follow it exactly. Do not invert the ratio or substitute initial amounts for equilibrium concentrations without accounting for the volumes.
Pause and explain
Which solvent order does this course use unless stated otherwise?
Worked example
At equilibrium, a solute has concentration 0.40 mol dm⁻³ in an organic solvent and 0.10 mol dm⁻³ in water. Find Kpc using the course convention.
Show the worked solution
- Write the stated organic-over-aqueous convention.
- Substitute equilibrium concentrations.
- Check units cancel.
Answer Kpc = [solute]organic/[solute]aqueous = 0.40/0.10 = 4.0.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
At equilibrium [solute]organic=0.60 mol dm⁻³ and [solute]aqueous=0.15 mol dm⁻³. Calculate Kpc.
Give me a hint
Use organic over aqueous concentration.
Compare my reasoning
- Write Kpc = [org]/[aq].
- Substitute 0.60 and 0.15.
- Cancel matching concentration units.
Kpc = 0.60/0.15 = 4.0.
Look for these in your work
- Convention followed
- Concentrations used
- Ratio calculated
A question defines Kpc as [solute]aqueous/[solute]organic. The concentrations are 0.12 mol dm⁻³ aqueous and 0.48 mol dm⁻³ organic. Find Kpc for that question.
Give me a hint
Follow the question's explicit convention even when it differs from the course default.
Compare my reasoning
- Write aqueous over organic.
- Substitute 0.12/0.48.
- State the dimensionless ratio.
Kpc = 0.12/0.48 = 0.25 using the question's convention.
Look for these in your work
- Question convention obeyed
- Equilibrium concentrations used
- Ratio stated
Explain why a mass ratio alone is not enough to compare partition coefficients when the solvent volumes differ.
Give me a hint
A partition coefficient compares concentration, which includes volume.
Compare my reasoning
- Concentration depends on amount divided by volume.
- Unequal phase volumes can give different amounts at the same concentration ratio.
- Use equilibrium concentrations in the defined order.
A mass or amount ratio ignores the solvent volumes. Kpc is formed from equilibrium concentrations, so each phase's volume must be included when converting measured amounts.
Look for these in your work
- Concentration meaning stated
- Volume effect explained
- Equilibrium requirement stated
Common mistakes
- Reversing the solvent order
- Using amounts rather than concentrations
- Comparing different chemical species
- Ignoring the equilibrium condition or temperature
What can you explain now?
What does a partition coefficient compare?
Compare with the explanation
Equilibrium concentrations of the same solute in two solvents
The ratio describes distribution of one species between immiscible phases.
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Make it stick
What must be equal or explicitly specified before a concentration ratio can represent a partition coefficient?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.