Summary

When chemists measure out an amount of a substance, they use an amount in moles. The mole is a useful quantity because one mole of a substance always contains the same number of entities of the substance. An amount in moles can be measured out by mass in grams, by volume in dm3 of a solution of known concentration and by volume in dm3 of a gas.

Specification

3.1.2.1 Relative atomic mass and relative molecular mass

Relative atomic mass and relative molecular mass in terms of 12C.

The term relative formula mass will be used for ionic compounds.

Students should be able to:

• define relative atomic mass (Ar)

• define relative molecular mass (Mr).

3.1.2.2 The mole and the Avogadro constant

The Avogadro constant as the number of particles in a mole.

The mole as applied to electrons, atoms, molecules, ions, formulas and equations.

The concentration of a substance in solution, measured in mol dm–3.

Students should be able to carry out calculations:

• using the Avogadro constant

• using mass of substance, Mr, and amount in moles

• using concentration, volume and amount of substance in a solution.

Students will not be expected to recall the value of the Avogadro constant.

3.1.2.3 The ideal gas equation

The ideal gas equation pV = nRT with the variables in SI units.

Students should be able to:

• use the equation in calculations.

Students will not be expected to recall the value of the gas constant, R.

3.1.2.4 Empirical and molecular formula

Empirical formula is the simplest whole number ratio of atoms of each element in a compound.

Molecular formula is the actual number of atoms of each element in a compound.

The relationship between empirical formula and molecular formula.

Students should be able to:

• calculate empirical formula from data giving composition by mass or percentage by mass

• calculate molecular formula from the empirical formula and relative molecular mass.

3.1.2.5 Balanced equations and associated calculations

Equations (full and ionic).

Percentage atom economy is:

molecular mass of desired product

sum of molecular masses of all reactants × 100

Economic, ethical and environmental advantages for society and for industry of developing chemical processes with a high atom economy.

Students should be able to:

• write balanced equations for reactions studied

• balance equations for unfamiliar reactions when reactants and products are specified.

Students should be able to use balanced equations to calculate:

• masses

• volumes of gases

• percentage yields

• percentage atom economies

• concentrations and volumes for reactions in solutions.

Required practical 1

Make up a volumetric solution and carry out a simple

acid–base titration.

Notes