Chemistry, Third edition

11 · SOLUTIONS AND SOLUBILITY

Solubility product of sparingly soluble salts

Experiments show that, for a saturated solution of silver chloride,

[Ag(aq)][Cl(aq)]a constant at that temperature

where the square brackets signify equilibrium concentrationsin mol dm^3. This is the

solubility product expressionfor silver chloride. The constant is known as the solu-

bility productorsolubility constant(symbolizedKs) for AgCl(s) at that temperature.

At 25C,Ks(AgCl)1.6 10 ^10 mol^2 dm^6 , i.e.

[Ag(aq)][Cl(aq)]Ks(AgCl)1.6 10 ^10 mol^2 dm^6

We can put this into words as follows: ‘In a solution saturated with AgCl at 25C, the

concentrations of silver and chloride ions when multiplied together must equal 1.6

 10 ^10 mol^2 dm^6 ’.

We can define the solubility products of other sparingly soluble ionic substances

in the same way:

1.For barium sulfate the equilibrium is

BaSO 4 (s) Ba^2 (aq) + SO 42 (aq)

Ks(BaSO 4 )[Ba^2 (aq)][SO 42 (aq)]1.1 10 ^10 mol^2 dm^6

2.For magnesium fluoride the equilibrium is

MgF 2 (s) Mg^2 (aq) + 2F(aq)

Ks(MgF 2 )[Mg^2 (aq)][F(aq)]^2 6.4 10 ^9 mol^3 dm^9

Note that the fluoride ion concentration is squaredin the last solubility product

expression because two Fions are produced when one MgF 2 (s) dissociates in

water.

The solubility products of selected compounds are listed in Table 11.4. Solubility

product expressions can be used only for sparingly soluble salts. They cannot be used

(without modification) for very soluble salts (like sodium chloride) because the con-

centration of ions is so high that the ions influence each other and the effective con-

centration of ions is lower than their concentration in mol dm^3. The errors in using

similar equations for slightly soluble ionic substances (such as calcium hydroxide)

are smaller, but are still significant in accurate work. (Now try Exercise 11E.)

178

Table 11.4Solubility products of

sparingly soluble ionic substances. The

units of concentration are mol dm^3.

Salt Ksat 25 °C

Silver chloride AgCl 1.6  10 ^10

Barium sulfate BaSO 4 1.1 10 ^10

Magnesium fluoride MgF 2 6.4 10 ^9

Calcium carbonate CaCO 3 8.7 10 ^9

Copper(I) iodide CuI 5.1  10 ^12

Iron(II) hydroxide Fe(OH) 2 1.6 10 ^14

Aluminium hydroxide Al(OH) 3 1.0 10 ^33

Zinc hydroxide Zn(OH) 2 2.0 10 ^27

Calcium sulfate CaSO 4 2.4 10 ^5

Lead(II) chloride PbCl 2 1.6 10 ^5

Solubility product

Write down solubility

product expressions for the

following sparingly soluble

compounds and state their

units:

(i)copper(I) iodide

(ii)aluminium hydroxide.

Exercise 11E