The Mole (Cambridge International IGCSE Chemistry)

The mole & the Avogadro Constant

Spec point — 3.2.1 · State that the mole, mol, is the unit of amount of substance and that one mole contains 6.02 × 1023 particles, e.g. atoms, ions, molecules; this number is the Avogadro constant

Extended tier only

• Chemical amounts are measured in moles

• The mole, symbol mol, is the SI unit of amount of substance

• One mole of a substance contains the same number of the stated particles

  • This can be atoms, molecules or ions

• One mole contains 6.02 x 10²³ particles; this number is known as the Avogadro Constant

• For example:

  • One mole of sodium (Na) contains 6.02 x 10²³ atoms of sodium
  • One mole of hydrogen (H₂) contains 6.02 x 10²³ molecules of hydrogen
  • One mole of sodium chloride (NaCl) contains 6.02 x 10²³ formula units of sodium chloride

• The mass of 1 mole of a substance is known as the molar mass

  • For an element, it is the same as the relative atomic mass written in grams
  • For a compound, it is the same as the relative molecular or formula mass in grams

The mole & volume of gas

Spec point — 3.2.1 · Use the molar gas volume, taken as 24dm3 at room temperature and pressure, r.t.p., in calculations involving gases

Extended tier only

Molar volumes of gas

• Avogadro’s Law states that at the same temperature and pressure, equal amounts of gases occupy the same volume of space

  • e.g. 1 mole of hydrogen gas occupies the same volume as 1 mole of methane gas

• At room temperature and pressure, the volume occupied by one mole of any gas was found to be 24 dm³ or 24,000 cm³

  • This is known as the molar gas volume at RTP
  • RTP stands for “room temperature and pressure” and the conditions are 20 ºC and 1 atmosphere (atm)

• From the molar gas volume, the following formula triangles can be derived:

Molar gas volume (dm³) formula triangle

This shows the relationship between moles of gas, volume in dm³and the molar volume

• If the volume is given in cm³ instead of dm³, then divide by 24,000 instead of 24:

Molar gas volume (cm³) formula triangle

This shows the relationship between moles of gas, volume in cm³and the molar volume

• The formula can be used to calculate the number of moles of gases from a given volume or vice versa

  • Simply cover the one you want and the triangle tells you what to do

• For example, to find the volume of a gas:

  • Volume = Moles x Molar Volume

Examples of Converting Moles to Volume Table

• For example, to find the number of moles of a gas:

  • Moles = Volume ÷ Molar Volume

Examples of Converting Volume to Moles Table

Exam Hint

• You are not expected to know the value of Avogadro's constant
• But, you do need to know the equation as well as how to use and re-arrange it