Practice question · Put in order
A precipitating reagent is added slowly to a solution containing two metal ions with very different solubility products. Order what happens.
- Both salts precipitate together and separation is lost
- The ion product of the less soluble salt reaches its solubility product first
- The more soluble salt reaches its own solubility product later
- The less soluble salt begins to precipitate
Hints
- The salt with the smaller solubility saturates first as the reagent is added.
- Selective precipitation works only while one salt alone is precipitating.
Show the answer
- The ion product of the less soluble salt reaches its solubility product first
- The less soluble salt begins to precipitate
- The more soluble salt reaches its own solubility product later
- Both salts precipitate together and separation is lost
Why
Adding the reagent slowly lets the less soluble salt saturate and precipitate on its own. Only once enough reagent has been added does the second salt reach its own constant, at which point the two co-precipitate and separation is lost, so the reagent is stopped before that.
Practise Solubility Equilibria
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