Chemistry, Third edition

CONCENTRATION OF SOLUTIONS

Ion concentrations

When ionic solids are dissolved in water, they are regarded as completely ionized in

solution. It is sometimes necessary to express the solution in terms of the concentra-

tions of the ions present, as shown in the examples below:

1.An aqueous NaCl solution, of concentration 1 mol dm^3 , contains Na+(aq) and

Cl(aq) ions. Each NaCl unit is dissociated in solution as follows:

NaCl(aq)Na(aq) + Cl(aq)

1.As 1 mol NaCl produces 1 mol Naand 1 mol Cl, a solution of NaCl of concen-

tration 1 mol dm^3 , contains a concentration of 1 mol dm^3 Na(aq) ions and

1 mol dm^3 Cl(aq) ions.

2.An aqueous CaCl 2 solution, of concentration 1 mol dm^3 , contains Ca^2 (aq)

and Cl(aq) ions. Each CaCl 2 unit is dissociated in solution as follows:

CaCl 2 (aq)Ca^2 (aq) + 2Cl(aq)

1.A solution of CaCl 2 of concentration 1 mol dm^3 , contains a concentration of

1 mol dm^3 Ca^2 (aq) and contains a concentration of 2 mol dm^3 Cl(aq).

3.Similarly, a 0.1 mol dm^3 solution of Na 3 PO 4 (aq) contains Na(aq) at a concen-

tration of 0.3 mol dm^3 and PO^34 (aq) at a concentration of 0.1 mol dm^3.

137

BOX 9.1

Concentrations of pure solids and pure liquids

Concentration is measured in mol dm^3. This is a measure of the number of particles per unit

volume of the substance. In pure solids or pure liquids the number of particles in a given

volume can be calculated from the density and this is constant, at constant temperature. Since

the density of a given pure solid or pure liquid is constant, at constant temperature, so is its

concentration.

1.The density of copper metal at 298 K is 8.92 g cm^3. This is equivalent to 8920 g dm^3 ,

or a concentration of 8920/63.5 mol dm^3 , which approximates to 140 mol dm^3.

2.The density of pure water at 298 K is 1 g cm^3 , equivalent to 1000 g dm^3 or a

concentration of 1000/18 mol dm^3. This approximates to 56 mol dm^3.

Concentration of solutions

(i) Sodium chloride (58.5 g) is dissolved in water. More water is then added so that the total

volume is 100 cm^3. What is the concentration of the solution in mol dm^3?

(ii)Calculate the concentration (in mol dm^3 ) of hydrated copper(II) sulfate (CuSO 4 ·5H 2 O) in a

solution originally prepared by dissolving 0.1 g of the solid in water and making up to

50 cm^3 with water.

(iii)What volume of sodium chloride solution of concentration 0.2 mol dm^3 contains 10 mol of

salt?

(iv)What mass of hydrated sodium carbonate (Na 2 CO 3 · 10H 2 O) should be dissolved in water

and made up to exactly 1 dm^3 in order to make the solution a concentration of

0.100 mol dm^3?

Exercise 9A