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Solubility Formula

The term solubility is all about to measure the quantity of solute dissolved in a particular solvent. The simple definition of solubility is solubility of a substance is the molarity of that substance in a solution which is at chemical equilibrium under excessive undissolved substance. This states that there must be a uniform temperature throughout the system, since solubility is temperature dependent.The Solubility Formula can be mathematically expressed as



The expression for solubility product has the same general form than other equilibrium constant expressions and KSP is called solubility product constant ”

Example 1

Determine the solubility of calcium fluoride CaF2


The equilibrium for this salt is,

CaF2(s) Ca+(aq) + 2F(aq)

The Ksp expression is expressed as

Ksp = [Ca2+][F]2 = 4.0 times 10-11 M3

Let S be the solubility of the Ca2+ ion

Then 2S is  the solubility of the F ion.

Substitute S in the Ksp expression, we get

(S)(2S)2 = 4.0 × 1011

4S3 = 4 × 1011

S = 2.15 ×10-4M


Example 2

The molar solubility of tin iodide SnI2 is 1.28 x 10-2 mol/L. Calculate the Ksp of this compound.


The solubility equilibrium of SnI2 is

SnI2(s) Sn2+(aq) + 2I(aq)

The Ksp expression is written as

Ksp = [Sn2+][I]2

1 mol of SnI2 Produces 1.0 mol of Sn2+, but 2.0mol of I.

[Sn2+] = 1.28 × 10-2M

[I] = (2) × 1.28 × 10-2M

= 2.56 × 10-2M

Substitute these values in Ksp expression

Ksp = (1.28 ×× 10-2M)(2.56 ×× 10-2M)2

Therefore, Ksp = 8.4 × 10-6 M2

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