The Mole (Cambridge (CIE) IGCSE Chemistry)

The mole & 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

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

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

  • Moles = Volume ÷ Molar Volume

Examples of Converting Volume to Moles Table