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Question

A sodium hydroxide solution can be used to distinguish between iron (II)sulphate (or chloride) and iron (III) sulphate (or chloride) solutions. Give the colour of the precipitate formed with each solution?


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Solution

i. When Iron (II)sulphate react with sodium hydroxide

  • A dirty green precipitate is formed.
  • This precipitate indicates the presence of ferric hydroxide.
  • FeSO4+2NaOHFe(OH)2dirtygrenorecipitate+Na2SO4
  • It is not soluble in excess reagent.
  • It turns to reddish brown precipitate slowly due to the oxidation and form Fe(OH)3
  • 4Fe(OH)2+O2+2H2O4Fe(OH)3reddishbrownprecipitate

ii. When iron (III) chloride react with sodium hydroxide

  • A reddish brown precipitate of Fe(OH)3 is formed.
  • FeCl3+3NaOHFeOH3reddishbrownprecipitate+3NaCl
  • When excess reagent is added, no reaction takes place.


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