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A Level

Partition Equilibrium Calculator

A free teacher tool for modelling solvent extraction and showing why several smaller extractions are more effective than one extraction using the same total volume of solvent.

What the calculator does

  • Models the distribution of a solute between two immiscible solvents.
  • States the chosen partition-coefficient convention clearly.
  • Calculates the amount transferred during a single extraction.
  • Shows the amount remaining after every extraction stage.
  • Compares one extraction with several smaller extractions.
  • Displays the complete mathematical derivation.

The key idea

K = concentration in organic layer ÷ concentration in aqueous layer

Each fresh portion of solvent establishes a new equilibrium. Repeated extraction therefore removes more solute than one extraction using the same total solvent volume.

Open the calculator

Enter the partition coefficient, solvent volumes and number of extractions.

Launch Partition Equilibrium Calculator

The calculator opens in a new browser tab.

For structured A-level chemistry teaching and guided mastery, visit ACE Chemistry Academy.

How to Use the Partition Equilibrium Calculator

The calculator compares the extraction of a solute between two immiscible solvents. It can model either a single extraction or several smaller extractions.

1. Check the definition of the partition coefficient

The calculator uses:

K = concentration in the organic layer ÷ concentration in the aqueous layer

Always check the order of the two concentrations. Reversing them gives a different value of K and an incorrect result.

A large value of K means that the solute has a greater preference for the organic solvent.

2. Enter the starting information

Supply the requested values:

  • the initial mass or amount of solute;
  • the volume of the aqueous solution;
  • the volume of organic solvent;
  • the partition coefficient, K;
  • the number of extractions, if more than one is being performed.

The two solvent volumes may be entered in cm³, provided the same unit is used for both.

3. Select the extraction method

Choose between:

  • one extraction using the complete volume of solvent;
  • several extractions using smaller portions of solvent.

For repeated extractions, check whether the volume entered is the volume used for each extraction or the total volume being divided between the extractions.

4. Follow the single-extraction calculation

At equilibrium:

K = [solute]organic ÷ [solute]aqueous

Because concentration is amount divided by volume, the calculator uses both the partition coefficient and the solvent volumes to determine:

  • the amount remaining in the aqueous layer;
  • the amount transferred to the organic layer;
  • the percentage of solute extracted.

For one extraction:

fraction remaining in the aqueous layer
= Vaqueous ÷ (Vaqueous + K × Vorganic)

The fraction extracted is:

1 − fraction remaining

5. Follow a multiple-extraction calculation

After each extraction, only a fraction of the solute remains in the aqueous layer.

If equal fresh portions of organic solvent are used:

fraction remaining after N extractions
= [Vaqueous ÷ (Vaqueous + K × Vorganic)]ᴺ

Here, (V_{\text{organic}}) is the volume used in each extraction.

The calculator applies the same equilibrium repeatedly and displays the amount remaining after each stage.

6. Compare the methods

Several smaller extractions are normally more effective than one extraction using the same total volume of solvent.

The advantage arises because each fresh portion of solvent establishes a new partition equilibrium and removes a fraction of the solute still remaining.

Check the reasoning

Before accepting the answer, check that:

  • the two solvents are immiscible;
  • the partition coefficient has been entered in the correct order;
  • the same volume unit has been used for both solvents;
  • the volume per extraction has not been confused with the total solvent volume;
  • the correct number of extractions has been entered;
  • the amounts in both layers add up to the original amount of solute.

The partition coefficient describes an equilibrium. The solute distributes between both layers—it is not normally transferred completely into one solvent.

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