The mole - Higher
Introducing the mole, the unit of measurement for the number of particles in a substance
The actual massThe amount of matter an object contains. Mass is measured in kilograms (kg) or grams (g). of atomThe smallest part of an element that can exist., moleculeA collection of two or more atoms held together by chemical bonds. and ionElectrically charged particle, formed when an atom or molecule gains or loses electrons. are too small to be useful in calculations. For this reason, chemical amounts are measured in moleThe amount of substance that contains the same number of particles as there are atoms in 12 g of carbon-12 (contains the Avogadro's constant 6.0 ×10²³ number of particles).. The mole is a unit. Its symbol is mol.
The mass of one mole of a substance is equal to:
- the relative atomic massThe mean relative mass of the atoms of the different isotopes in an element. It is the number of times heavier an atom is than one-twelfth of a carbon-12 atom. (Ar) of its formula in grams if the substance is an elementA substance made of one type of atom only.
- the relative formula massThe sum of the relative atomic masses of the atoms in a chemical formula. (Mr) of its formula in grams if the substance is a compoundA substance formed by the chemical union of two or more elements.
The table shows the masses of one mole of three substances.
| Substance | Formula | Relative formula mass | Mass of 1 mol |
| Calcium | Ca | 40 | 40 g |
| Oxygen | O2 | 2 Ă— 16 = 32 | 32 g |
| Calcium carbonate | CaCO3 | 40 + 12 + (3 Ă— 16) = 100 | 100 g |
| Substance | Calcium |
|---|---|
| Formula | Ca |
| Relative formula mass | 40 |
| Mass of 1 mol | 40 g |
| Substance | Oxygen |
|---|---|
| Formula | O2 |
| Relative formula mass | 2 Ă— 16 = 32 |
| Mass of 1 mol | 32 g |
| Substance | Calcium carbonate |
|---|---|
| Formula | CaCO3 |
| Relative formula mass | 40 + 12 + (3 Ă— 16) = 100 |
| Mass of 1 mol | 100 g |
Avogadro constant
One mole of a substance contains the same number of particleA general term for a small piece of matter. For example, protons, neutrons, electrons, atoms, ions or molecules. as one mole of any other substance. The particles can be atoms, molecules or ions.
The number of atoms, molecules or ions in one mole of a substance is called the Avogadro constantThe number of particles in one mole of particles, 6.02 × 10²³ per mole.. Its value is 6.02 × 1023 per mole, which is 602,000,000,000,000,000,000,000 per mole.
The amount in moles can apply to atoms, molecules, ions and electrons. For example, the number of atoms in 1 mol of sulfur is the same as the number of molecules in 1 mol of sulfur dioxide. This is the same as the number of sodium ions in 1 mol of sodium chloride. The number of chloride ions in 1 mol of sodium chloride is also the same.
Calculating the number of particles
The number of particles in a substance can be calculated using:
- the Avogadro constant (6.02 Ă— 1023)
- the amount of substance in mol
Number of particles = Avogadro constant Ă— the amount of substance in mol
Worked example
Calculate the number of water molecules in 0.5 mol of water.
Number of water molecules = Avogadro constant x amount of substance in mol
= 6.02 Ă— 1023 Ă— 0.5
= 3.01 Ă— 1023
It is important to state the particles involved. For example, 3.01 Ă— 1023 water molecules contain 9.03 Ă— 1023 atoms. This is because one water molecule, H2O, is made up of three atoms.
Question
Calculate the number of oxygen atoms in 0.5 mol of oxygen molecules, O2.
Number of atoms = 6.02 Ă— 1023 per mol Ă— 0.5 mol Ă— 2
= 6.02 Ă— 1023