Chemistry, Third edition

Calculating Concentrations

Objectives

Describes how the concentrations of solutions can be expressed in

appropriate units

Gives examples of how volumetric analysis can be used to find the

concentration of solutions

Explains what is meant by ‘pH’

Concentration of solutions

We have already discussed how to calculate masses of solids or volumes of gases, for

the reactants or products in balanced equations. What if solutions are involved in

the reaction? An aqueous solution may be concentrated or dilute. That is, it may

contain a large or small amount of solid dissolved in a given amount of water as

shown in Fig. 9.1.

How can we express the concentrations of the solutions?

Molar concentration

It is common to measure the concentration of a solution in moles per cubic decimetre

(mol dm^3 ). For example, a solution of sodium chloride, of concentration

1 mol dm^3 , contains 1 mol (or a mass of 23 + 35.5 58.5 g) of NaCl dissolved in

1dm^3 ofsolution.Always use the term ‘solution’ and not water.A mixture of 58.5 g of

NaCl dissolved in 1 dm^3 of water is notquite the same thing.

Different textbooks, according to their age and their country of origin, may

express concentration as M (molar) or mol L^1 (moles per litre). Both these units

may be taken to mean the same as mol dm^3.

Some examples of molar concentration are:

1.A solution of sodium chloride of concentration 2.00 mol dm^3 contains 2.00 mol

(or 2.00 58.5117 g) of NaCl dissolved in 1 dm^3 of solution.

2.Sodium chloride solution of concentration 0.500 mol dm^3 contains 0.500 mol

(or 0.500 58.529.3 g) of NaCl dissolved in 1 dm^3 of solution.

9.1

Contents

9.1Concentration of

solutions 135

9.2Standard solutions 138

9.3Volumetric analysis 140

9.4Other units of

concentration 146

9.5pH scale 150

9.5Revision questions 152

Dilute Concentrated

Sugar

molecules

Fig. 9.1Sugar solutions at

two different concentrations.

9

UNIT