# 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

$\dpi{120}&space;\large&space;S=\sqrt{K_{sp}}$

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

Solution:

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.

Solution:

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