Chemistry, Third edition

The Mole

Objectives

Explains why chemists work in moles

Describes how to perform chemical calculations, involving masses and gas

volumes using moles

Defines ‘percentage yield’

Explains how to work out which substance is the limiting reagent in a

chemical reaction

Molecular mass

The idea of molecular masswas introduced in Chapter 3. This is the mass of one

molecule of substance on the atomic mass scale. Molecular masses are calculated

using the atomic masses of the constituent atoms. A list of approximate atomic

masses for atoms of elements is shown in Table 8.1. You should use these for calcula-

tions, unless you are instructed otherwise.

Examples of calculations of molecular mass

m(H 2 O) 1  1  16 18 u

m(C 6 H 5 Cl)(612)(51)(35.5)112.5 u

m(H 2 SO 4 )(21)(32)(416)98 u

Substances such as sodium chloride (Na+, Cl) and copper(II) nitrate (Cu2+, 2NO 3 )

consist of ions (not molecules) and the term ‘molecular mass’ is not strictly appro-

priate. Nevertheless the ‘formula mass’ of these substances are calculated in a similar

way to neutral molecules. Examples are

m(Na+, Cl) 23 35.558.5 u

m(Cu2+, 2NO 3 )(63.5)2(14 16  16 16)187.5 u

8.1

Contents

8.1Molecular mass 118

8.2Moles 119

8.3Percentage
composition by mass 124

8.4Water of
crystallization 127

8.5Calculating amounts
from equations 127

8.6Calculating gas
volumes 129

8.7Percentage yield 130

8.8Limiting reagents 131

8.8Revision questions 133

8

UNIT