The Mole (CIE IGCSE Chemistry: Co-ordinated Sciences (Double Award))

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

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 dm³ and 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 cm³ and 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

Gas	Amount (moles)	Volume
Hydrogen	3	(3 x 24) = 72 dm3  (3 x 24000) = 72000 cm3
Carbon dioxide	0.25	(0.25 x 24) = 6 dm3  (0.25 x 24000) = 6000 cm3
Oxygen	5.4	(5.4 x 24) = 129.6 dm3  (5.4 x 24000) = 129600 cm3
Ammonia	0.02	(0.02 x 24) = 0.48 dm3  (0.02 x 24000) = 480 cm3

 

• For example, to find the number of moles of a gas:
  • Moles = Volume ÷ Molar Volume

Examples of Converting Volume to Moles Table

Gas	Volume	Moles
Methane	225.6 dm3	(225.6 ÷ 24) = 9.4 mol
Carbon monoxide	7.2 dm3	(7.2 ÷ 24) = 0.3 mol
Sulfur dioxide	960 dm3	(960 ÷ 24) = 40 mol
Oxygen	1200 cm3	(1200 ÷ 24000) = 0.05 mol