Chemistry, Third edition

SOLUBILITY OF SPARINGLY SOLUBLE IONIC COMPOUNDS 181

Example 11.2

25.0 cm^3 of a 0.0020 mol dm^3 solution of iron(II) sulfate is added

to 25.0 cm^3 of a 0.0040 mol dm^3 solution of sodium hydroxide.

Will iron(II) hydroxide precipitate?

Answer

For iron(II) hydroxide

Ks(Fe(OH) 2 )[Fe^2 (aq)][OH(aq)]^2

(Notice that the hydroxide ion concentration is squared because iron(II) hydroxide

produces two hydroxide ions when it dissociates.)

total volume of solution 25.0 cm^3 25.0 cm^3 50.0 cm^3

Iron(II) sulfate FeSO 4 ; the concentration of Fe^2 (aq) ion will be

0.0010 mol dm^3 because of the dilution from 25.0 cm^3 to 50.0 cm^3.

Sodium hydroxide NaOH; the concentration of OH(aq) ion will be

0.0020 mol dm^3 because of the dilution from 25 cm^3 to 50 cm^3.

The product of the ion concentrations is then

[Fe^2 (aq)][OH(aq)]^2 (0.0010)(0.0020)^2 4.0 10 ^9 mol^3 dm^9

This is very much greater than the solubility product of Fe(OH) 2 (1.6

10 ^14 mol^3 dm^9 , Table 11.4) and so iron(II) hydroxide will precipitate out.

Fig. 11.4Common ion effect.

Precipitation

(i)A solution of volume 10.0 cm^3 contains magnesium ions at a concentration of

0.0010 mol dm^3 , and fluoride ions at a concentration of 0.0030 mol dm^3. Will

magnesium fluoride precipitate? What happens if 90 cm^3 of pure water is then added to the

solution?

(ii)5.0 cm^3 of 2.0  10 ^4 mol dm^3 barium chloride is mixed with 5.0 cm^3 of 4.0 

10 ^4 mol dm^3 sodium sulfate solution. Will barium sulfate precipitate out?

Exercise 11G

Cloudiness

seen as NaCl

precipitates

Concentrated

HCl (aq)

Saturated

sodium

chloride solution