The Mole (CIE IGCSE Chemistry)

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The Mole & Avogadro's Constant

• 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, atoms, molecules, or ions as one mole of any other substance
  
• One mole contains 6.02 x 10²³ particles (e.g. atoms, ions, molecules); 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 formula mass or relative molecular mass in grams

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• Avogadro’s Law states that at the same conditions of temperature and pressure, equal amounts of gases occupy the same volume of space
• 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 triangle can be derived:

 

Formula triangle showing 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:

Formula triangle showing 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

To find the volume of a gas

Volume = Moles x Molar Volume

Examples of Converting Moles into Volumes Table

 

To find the moles of a gas

Moles = Volume ÷ Molar Volume

Examples of Converting Volumes into Moles Table