Chemistry, Third edition

SOLUBILITY OF SPARINGLY SOLUBLE IONIC COMPOUNDS

not hold for very soluble salts, where the presence of appreciable concentrations of

salt in the water causes a slight change in the liquid volume.

One reminder – although many ionic substances are only sparingly soluble in

water, any ionic substance that doesdissolve completely dissociates into separate

ions.

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Example 11.1

The solubility of silver chloride is 1.8  10 ^3 g of AgCl per 1000 g

of saturated AgCl solution at 25C. (a) Express this solubility in

mol of AgCl per dm^3 of AgCl solution. (b) What is the

concentration of Ag(aq) and Cl(aq) (in mol dm^3 ) in a

saturated solution of AgCl? (Assume that 1 cm^3 of solution has a

mass of exactly 1 g at 25C.)

Answer

(a) The molar mass of AgCl M(AgCl)143.5 g mol^1. Therefore, a mass of 1.8

 10 ^3 g contains

1.8 10 ^3 /143.51.3 10 ^5 mol AgCl

The solution, which contains 1000 g of water, has a volume of 1000 cm^3 

1.000 dm^3. Therefore, the molar solubility of AgCl is

s

amount of substance (in moles)

volume

s

1.3 10 ^5

1.3 10 ^5 mol dm^3

1.000

(b) The dissolution of silver chloride is represented by the equation

AgCl(s) Ag(aq) + Cl(aq)

(i.e. Ag,Cl)

The Ag,Clpairs that do dissolve, dissociate completely into ions. Since one pair

of dissolved Ag,Clions produces one Agion and one Clion, the

concentration of these ions is also 1.3  10 ^5 mol dm^3 :

[Cl(aq)][Ag(aq)]1.3 10 ^5 mol dm^3

Molar solubility

Lead(II) chloride (PbCl 2 ) has a molar solubility at 25C of

1.60 10 ^2 mol dm^3 of solution. What mass of PbCl 2

does 100 cm^3 of water at 25C which is saturated with

PbCl 2 contain? What is the concentration of lead and

chloride ions (in mol dm^3 ) in the saturated solution?

Exercise 11D