Empirical Formula and Water of Crystallisation

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Molecular and Empirical Formula

'The molecular formula of a substance shows the number of atoms of each element in one molecule of the substance'

'The empirical formula of a substance shows the simplest whole number ratio of atoms of each element present in a substance'

Formulae of Hydrated Salts

  • When you have some compounds dissolved in water and then the water evaporates, you are left with crystals
  • The crystals don't feel wet, but sometimes there are water molecules left, locked around the ions in the crystal
  • This water is called the water of crystallisation and is part of the solid structure of the compound
  • A salt which includes dome water of crystallisation is called a hydrated salt
  • If a hydrated salt is heated, the water may be driven off as steam, leaving the salt behind but now with no water of crystallisation. This is called an anhydrous salt
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