Linking Moles, Mass and Mr

Section 1 of 7The mole and the Avogadro constant (HT only)

The mole

• The mole The mole (symbol mol) is the SI unit for amount of substance.
  • It is a counting unit, like a dozen, but for extremely small particles.
• One mole of any substance contains the same number of the stated particles, whether they are atoms, molecules, ions or formula units.

Avogadro constant

• Avogadro constant One mole contains 6.02 × 10²³ particles. This number is the Avogadro constant.
• You must say which particle you mean: 1 mol of Na is 6.02 × 10²³ atoms, 1 mol of H₂ is 6.02 × 10²³ molecules and 1 mol of NaCl is 6.02 × 10²³ formula units.

Section 2 of 7Molar mass (HT only)

Elements

• Elements One mole of an element has a mass in grams equal to its A_r.
  • So 1 mol of carbon is 12 g, 1 mol of helium is 4 g and 1 mol of lithium is 7 g.

Compounds

• Compounds One mole of a compound has a mass in grams equal to its M_r.
  • For carbon dioxide M_r = 12 + (2 × 16) = 44, so 1 mol has a mass of 44 g.
• The mass of one mole is called the molar mass and has units g/mol.

Section 3 of 7The key equation (HT only)

To find	Equation	Units
Moles	n = m ÷ Mr	mol
Mass	m = n × Mr	g
Mr (molar mass)	Mr = m ÷ n	g/mol
Number of particles	N = n × 6.02 × 1023	particles

Section 4 of 7Worked example: mass from moles (HT only)

1

Question: what is the mass of 0.250 mol of zinc?

2

Look up the A_r of zinc: 65, so the molar mass is 65 g/mol.

3

Rearrange the equation: mass = moles × molar mass.

4

Mass = 0.250 mol × 65 g/mol = 16.25 g, which is 16.3 g to three significant figures.

Section 5 of 7Worked example: moles and particles (HT only)

1Moles from mass

• Moles from mass How many moles are in 2.64 g of sucrose (M_r = 342)? Moles = 2.64 ÷ 342 = 7.72 × 10^-3 mol.

2Moles to molecules

• Moles to molecules For 15.7 g of water (M_r = 18): moles = 15.7 ÷ 18 = 0.872 mol.

3Number of molecules

• Number of molecules 0.872 × 6.02 × 10²³ = 5.25 × 10²³ molecules.

4Number of atoms

• Number of atoms Each water molecule has 3 atoms, so there are 3 × 5.25 × 10²³ = 1.57 × 10²⁴ atoms.

Section 6 of 7Key terms

Term	Meaning
Mole	The unit for amount of substance; one mole contains 6.02 × 1023 particles.
Avogadro constant	The number of particles in one mole of a substance, 6.02 × 1023 per mole.
Molar mass	The mass of one mole of a substance, in g/mol, numerically equal to its Ar or Mr.
Formula unit	The simplest ratio of ions in an ionic compound, such as one NaCl.
Amount of substance	A measure of how many particles are present, measured in moles.

Section 7 of 7Exam technique

1

Multiplying by 6.02 × 10²³ without first saying whether you are counting atoms, molecules or ions; count atoms by multiplying by the atoms in the formula.

2

Using mass in kilograms in the equation; the molar mass is in g/mol so the mass must be in grams.

3

Rearranging the equation the wrong way round; check that your answer is sensible, since a heavy sample should give a number of moles that is smaller than the mass.

4

Writing only the final answer with no working, which loses method marks if a slip occurs.