The Mole (Oxford AQA IGCSE Combined Science Double Award)

The Mole

• One mole of any element is equal to the relative atomic mass of that element in grams

• If you had 1 mole of carbon atoms in your hand, that number of carbon atoms would have a mass of 12 g (because the A_r of carbon is 12)

  • So, one mole of helium atoms has a mass of 4 g (A_r of He is 4)

  • One mole of lithium has a mass of 7 g (A_r of Li is 7) and so on

• One mole of any compound is the relative molecular mass or relative formula mass in grams

• To find the mass of one mole of a compound, add up the relative atomic masses

  • For example, carbon dioxide has an Mr of:
    (1 x C) + (2 x O)
    (1 x 12) + (2 x 16) = 44

Moles, mass and relative mass

• The number of moles of any chemical can be calculated using:

Moles =

• We can use the following formula triangle to convert between moles, mass in grams and the molar mass:

Formula triangle for mass (g), moles and relative formula mass

• Calculating the number of moles of an element uses the same equation, but with relative atomic mass replacing M

Calculating Moles & Masses

• Chemical amounts are measured in moles

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

• One mole of any 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