Solubility Product and Solubility of a Sparingly Soluble Salt

In this video we want to discuss how to determine solubility of a sparingly soluble salt from its solubility product Ksp.

But first we need to establish the fact that Ksp and solubility are both related to a saturated solution.

Solubility Product Ksp and Saturated Solution

When a salt dissolves and the solution is diluted, the concentration of ions is expected to continue to increase.

This means the system is not at equilibrium and equilibrium constant Ksp is not relevant for a diluted solution.

When the solution is saturated, the concentration of ions is at its maximum and constant.

Hence the system is now at equilibrium and Ksp is relevant for a saturated solution.

Solubility and Saturated Solution

Solubility is the amount of salt that dissolves to give a saturated solution.

Therefore solubility is directly related to a solution that is saturated.

Since both Ksp and solubility are related to a saturated solution, this means Ksp and solubility are related to each other, and we can use one of the value to find the other.

Ksp and Solubility relationship with saturated solution

Dissocation of Sparingly Soluble Salt CaF2

Let's use CaF2 as an example.

Ksp and Solubility dissociation of salt CaF2

When CaF2 is put in a beaker of water, it'll dissociate partially to form Ca2+ and F- ions.

We can represent this using the following equation.

Ksp and Solubility dissociation equation

If we let x be the solubility of CaF2, x moles of CaF2 will dissolve to give a saturated solution which contains x moles of Ca2+ and 2x moles of F-.

Ksp and Solubility concentration at equilibrium

We can now write out the solubility product and express this in terms of solubility x to work out the relationship between Ksp and x.

Ksp and Solubility Ksp and solubility working

This means that if Ksp is given, we can determine solubility of the salt, and vice versa.

For the detailed step-by-step discussion on how to determine solubility of a sparingly soluble salt from its solubility product Ksp, check out this video!

Topic: Solubility Product, Physical Chemistry, A Level Chemistry, Singapore

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