Common Ion Effect Calculator
How far a shared ion suppresses solubility.
Work out Common Ion Effect. How far a shared ion suppresses solubility. Kelvin conversion handled for you.
The anion concentration from another source — 0.1 M NaCl gives 0.1 M chloride.
Solubility with the common ion
1.800e-9 M
7,454× less soluble than in pure water
Le Chatelier's principle in its most practical form: adding an ion that is already on the right of the dissolution equation pushes the equilibrium back and the salt dissolves less. Silver chloride is thousands of times less soluble in 0.1 M sodium chloride than in water, which is exactly why gravimetric analysis washes precipitates with a dilute solution of the common ion rather than pure water — plain water would redissolve part of the sample you are trying to weigh. The equation has no closed-form solution for the general case, so this solves it numerically rather than using the usual textbook approximation.
How the Common Ion Effect Calculator works
Enter a Ksp and the concentration of an ion already present in solution, and this returns the suppressed solubility alongside the value in pure water and the ratio between them. It solves the equilibrium numerically rather than using the usual approximation, so it stays correct even when the common ion is dilute.
Also known as: solubility with a common ion · ksp with common ion calculator · le chatelier solubility calculator · solubility suppression calculator
Frequently asked questions
What is the common ion effect?
A salt is less soluble in a solution that already contains one of its ions. Adding the ion pushes the dissolution equilibrium backwards, exactly as Le Chatelier's principle predicts.
How much less soluble does it get?
Often by orders of magnitude. Silver chloride's solubility drops from 1.3 × 10⁻⁵ M in water to 1.8 × 10⁻⁹ M in 0.1 M sodium chloride — a factor of about 7,000.
Why do labs wash precipitates with a dilute salt solution?
To stop the precipitate redissolving. Rinsing with pure water dissolves part of what you are trying to weigh; rinsing with a solution containing the common ion removes impurities while leaving the precipitate intact.
Does the common ion effect apply to weak acids too?
Yes, and it is how buffers work. Adding sodium acetate to acetic acid suppresses the acid's ionisation, which is the common ion effect under a different name.
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