Introduction:

Knowing how to calculate the solubility product constant (Ksp) is essential for understanding various aspects of chemistry, including equilibrium, saturation, and precipitation. In this article, we will discuss how to calculate Ksp from solubility using a step-by-step procedure.

Step 1: Understand the basics of Ksp

The solubility product constant (Ksp) represents the equilibrium between the dissolved ions and the undissolved solid in a solution. It is a value unique to each substance and is dependent on temperature. The higher the Ksp value, the higher the solubility of the substance in water.

Step 2: Write down the chemical equation

To begin with, you need to write the balanced dissociation equation for the ionic compound in water. The equation should represent the dissolution of one mole of the solid into its separate ions.

For example, consider a sparingly soluble salt like calcium sulfate (CaSO4). The balanced dissociation equation for this salt can be written as:

CaSO4(s) ⇌ Ca²⁺(aq) + SO₄²⁻(aq)

Step 3: Define the solubility

The solubility of an ionic compound is defined as the maximum concentration of dissolved ions in a solution, at a specific temperature. It is typically expressed in terms of moles per liter (mol/L).

Step 4: Establish relationships between ions’ concentrations

One mole of CaSO₄ dissociates into one mole of Ca²⁺ and one mole of SO₄²⁻. Thus, if ‘s’ represents the solubility of CaSO4, then:

[Ca²⁺] = s

[SO₄²⁻] = s

Step 5: Set up and solve for Ksp

Now that we have the equilibrium concentrations for each ion, we can substitute these values into the Ksp expression:

Ksp = [Ca²⁺]·[SO₄²⁻]

So, in terms of solubility ‘s’:

Ksp = s²

To calculate Ksp from solubility, simply find the solubility of the compound and square it.

Step 6: Example with given solubility

Imagine the solubility of CaSO4 is 4.93 x 10⁻³ mol/L. To find Ksp, substitute this value into the equation derived in

step 5:

Ksp = (4.93 x 10⁻³)²

Ksp ≈ 2.43 x 10⁻⁵

So, the solubility product constant for CaSO4 is approximately 2.43 x 10⁻⁵.

Conclusion:

Calculating Ksp from solubility involves understanding the dissociation equation and setting up a Ksp expression with correct concentrations. Once you know the solubility of a specific substance, it’s fairly simple to determine its Ksp value using this method. Keep in mind, though, that Ksp is temperature-dependent, so always make sure you’re working with values at the same temperature.