Difference Between Molar Solubility and Product Solubility Constant

The key difference between molar solubility and product solubility constant is that the molar solubility describes the dissolution of a substance per litre of a solution, whereas the product solubility constant describes the dissolution of a solid substance in an aqueous solution.

Both molar solubility and product solubility constant are chemical concepts that describe the dissolution of substances in solutions. We can calculate molar solubility from product solubility constant. Therefore, they are related to each other.

1. Overview and Key Difference
2. What is Molar Solubility
3. What is Product Solubility Constant
4. Side by Side Comparison – Molar Solubility and Product Solubility Constant in Tabular Form
5. Summary

What is Molar Solubility?

Molar solubility is the number of moles of a substance that dissolve per litre of a solution before saturation. That means; the molar solubility gives the amount of a substance that we can dissolve in a solution before the solution gets saturated from that particular substance. We can calculate this amount using the product solubility constant or Ksp and the stoichiometry. The unit for the molar solubility is mol/L. We can denote this term as “M”. We can calculate the molar solubility using Ksp, but we have to know the ions produced by the dissociation during the dissolution of the substance in the solution.

Let us consider an example; if AB is dissolved in an aqueous solution, it dissociates into A and B ionic products. The equation for this dissolution is as follows:

AB_(s) ⇌ A_(aq) + B_(aq)

According to the stoichiometry of this reaction, if the final concentration of A is “x”, then the final concentration of B is also “x”. Then, the solubility product equation for this reaction is;

Ksp = [A][B]

= [x][x]

= x²

Here, x is the molar solubility. Therefore, if we know the Ksp of the reaction, we can calculate the x, molar solubility of the reaction.

What is Product Solubility Constant?

Product solubility constant or solubility product constant is the equilibrium constant for the dissolution of a solid substance in an aqueous solution. Therefore, it describes the dissolution of a solid substance in an aqueous solution, and it represents the level at which a solute dissolves in a solution. We can denote it as Ksp. Generally, Ksp is calculated by multiplying the concentrations of ionic products after the dissolution of the substance. For the dissolution of AB₂, the reaction is as follows:

AB_2(s) ⇌ A_(aq) + 2B_(aq)

For the above reaction, the product solubility constant or Ksp is as follows:

Ksp = [A_(aq)][B_(aq)]²

What is the Difference Between Molar Solubility and Product Solubility Constant?

Both, the molar solubility and product solubility constant, describe the dissolution of a substance in a solution. However, the key difference between molar solubility and product solubility constant is that the molar solubility describes the dissolution of a substance per litre of a solution, whereas the product solubility constant describes the dissolution of a solid substance in an aqueous solution.

Besides, the molar solubility is the number of moles of a substance that dissolve per litre of a solution before saturation. Meanwhile, the product solubility constant is, or solubility product constant is the equilibrium constant for the dissolution of a solid substance in an aqueous solution. We can denote molar solubility as “M” and product solubility constant as “Ksp”.

Below infographic summarizes the difference between molar solubility and product solubility constant.

Summary – Molar Solubility vs Product Solubility Constant

In summary, both, the molar solubility and product solubility constant, describe the dissolution of a substance in a solution. However, the key difference between molar solubility and product solubility constant is that the molar solubility describes the dissolution of a substance per litre of a solution, whereas the product solubility constant describes the dissolution of a solid substance in an aqueous solution.

Reference:

1. “Solubility Product Constant, Ksp.” Chemistry LibreTexts, Libretexts, 5 June 2019, Available here.
2. “Molar Solubility.” Wikipedia, Wikimedia Foundation, 5 Apr. 2019, Available here.
3. “Solubility Equilibrium.” Wikipedia, Wikimedia Foundation, 26 Sept. 2019, Available here.