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Mole

• The term mole is derived from the word molecule.
  
  
• Like atoms, molecules are so small that a very large number of molecules are involved in reactions etc.
  
  
• The unit mole is used by chemists to represent a large number of molecules in the same way that a baker uses a dozen to represent 12 buns, or the way 12 dozen is represented by a gross.
  
  
• 1 mole of any substance is 6.0 x 10²³ molecules of that substance.
  
  
• 6.0 x 10²³ is called the Avagadro's number.
  
  
• Units such as a mole, atomic weight (AW), molecular weight (MW) or molar solution (M) are all based on the Avagadro's number, and are a convenient way of dealing with such a large number of molecules.
  
  
• 1 mole of any substance is its AW (atomic weight) or MW (molecular weight) in grams.
  
  
• As examples -
  
  
  • The molecular weight (MW) of sodium hydroxide is 40.
    
    Therefore 1 mole or 6.0 x 10²³ molecules of sodium hydroxide weighs 40 grams.
    
    (The very reason it was assigned a MW of 40)
    
    
  • A molecule of SO₂ is composed of one (1) sulphur (S) and two (2) oxygens (O).
    
    Hence 1 mole of SO₂ will contain 1 mole of sulphur (S) and 2 moles of oxygen (O)
    
    or 6.0 x 10²³ molecules of S and 12 x 10²³ molecules of O.