The Mole (AQA GCSE Chemistry)

The mole

Higher 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 and atomic mass

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

  • This is called the molar mass

• If you had 6.02 x 10²³ atoms of carbon 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 would have a mass of 4 g (A_r of He is 4), one mole of lithium would have a mass of 7 g (A_r of Li is 7) and so on

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

  • So one mole of water would have a mass of (2 x 1) + 16 = 18 g

Calculating moles & masses

Higher tier only

• Although elements and chemicals react with each other in molar ratios, in the laboratory we use digital balances and grams to measure quantities of chemicals as it is impractical to try and measure out moles

• Therefore, we have to be able to convert between moles and grams

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

Formula triangle for moles, mass and molar mass