DETERMINE THE MASS OF ALUM CRYSTALS

    THEORY

1. Aluminium metal is treated with hot aqueous KOH solution. Aluminium dissolves as potassium aluminate, KAl(OH)₄, salt.

               2Al(s) + 2KOH(aq) + 6H₂O(l) -> 2KAl(OH)₄(aq) + 3H₂(g)

2. Potassium aluminate solution on treatment with dil. sulphuric acid first gives ppt. of Al(OH)₃, which dissolves on addition of small excess of H₂SO₄ and heating.

               2KAl(OH)₄(aq) + H₂SO₄(aq) -> 2Al(OH)₃(s) + K₂SO₄(aq) + 2H₂O(l)

               2Al(OH)₃(s) + 3H₂SO₄(aq) -> Al₂(SO₄)₃(aq) + 6H₂O(l)

3. The resulting solution is concentrated to near saturation and cooled. On cooling crystals of potash alum crystallize out.

               K₂SO₄(aq) + Al₂(SO₄)₃(aq) + 24H₂O(l) ->K₂SO₄.Al₂(SO₄)₃.24H₂O(s)