12th Chemistry Projects for 2009 Exams
DETERMINE THE MASS OF ALUM CRYSTALS
Aluminium because of its
low density, high tensile strength and resistance to corrosion is widely used
for the manufacture of airplanes, automobiles, lawn furniture as well as for
aluminium cans. Being good conductor of electricity, it is also used for
transmission of electricity. Aluminium foil is used for wrapping cigarettes,
confectionery items, etc. Aluminium is also used for making utensils. The
recycling of aluminium cans and other aluminium products is a very positive
contribution to saving our natural resources. Most of the recycled aluminium is
melted and recast into other aluminium metal products or used in the production
of various aluminium compounds, the most common of which are the alums. Alums
are double sulphates having general formula X2SO4.M2(SO4)3.24H2O
X = monovalent cation
such as Na+, K+, NH4+, etc.
M = trivalent cation
such as Al+3, Cr+3, Fe+3, etc.
Some important alums and
their names are given below:
Potash Alum: K2SO4.Al2(SO4)3.24H2O
Soda Alum Na2SO4.Al2(SO4)3.24H2O
Chrome Alum K2SO4.Cr2(SO4)3.24H2O
Ferric Alum (NH4)2SO4.Fe2(SO4)3.24H2O
Alums are isomorphous
crystalline solids, which are soluble in water. Potash alum is used in paper
making, in fire extinguisher, in foodstuffs and in purification of water. Soda
alum is used in baking powders and chrome alum is used in tanning leather and
waterproofing fabrics. Ferric alum is used in antiseptics. The shape of a potash
alum crystal is octahedral.
- Conical flasks
- Filter paper
- Piece of aluminium foil
- Potassium Hydroxide (KOH)
- Sulphuric Acid (H2SO4)
- Ethanol (C2H5OH)
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)
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
+ H2SO4(aq) ->
2Al(OH)3(s) + K2SO4(aq) + 2H2O(l)
+ 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.
+ Al2(SO4)3(aq) + 24H2O(l) ->K2SO4.Al2(SO4)3.24H2O(s)
1. Prepare 50ml of 4M
2. Add small pieces of
aluminium foil (about 1 gm) in the conical flask containing the KOH solution.
Since during this step hydrogen gas is evolved, this step must be done in a
3. After all of
aluminium has reacted, filter the solution to remove any insoluble impurities.
4. Allow the filtrate to
cool. Now add slowly conc. H2SO4 until insoluble Al(OH)3
just forms in the solution
5. Gently heat the
mixture until the Al(OH)3 ppt. dissolves. Leave the solution
overnight for the crystallization to continue.
6. Take out the crystals
and wash them with 50/50 ethanol-water mixture.
7. Determine the mass of
the alum crystals.
Mass of aluminium metal
= 1 gm
Mass of potash alum = 13
Colour = White
Shape of crystals =
1. A few drops of conc.
sulphuric acid should be added while preparing saturated solution of aluminium
sulphate to prevent its hydrolysis.
2. Aluminium sulphate
solution should be clear and not turbid.
3. Cool the conc.
solution slowly to get large crystals. Rapid disturbance of solution may change
the shape, size and quantity of the crystals.
4. Conc. solution should
be cooled undisturbed. A slight disturbance of solution may change the shape,