Question

1. In preparation of common alum, why is it necessary to reduce the volume of your reaction mixture after the addition of sulfuric acid?

2. What was the whitish material that formed in the filter flask liquid after you began washing your product?

3. What was the purpose of washing your product with either ice-cold water or alcohol?

4. Why must the oven temperature be kept below 100 C when drying your product? (Two Reasons)

Answer 1

Preparation of alum from aluminium

1. In preparation of common alum, it is necessary to reduce the volume of your reaction mixture after the addition of sulfuric acid to enable crystalization of alum fromed in solution. Excess solution would dissolve more alum in it and product yield would be lower.

2. The whitish material that formed in the filter flask liquid after you began washing your product is Al(OH)3.

3. The purpose of washing your product with either ice-cold water or alcohol is to remove any excess H2SO4 present in the crystals. Alum is soluble in water at room temperature and therefore ice cold solution is used for washing so no product is lost in washing stage.

4. The oven temperature be kept below 100 C when drying your product as higher temperature would remove water of crystallization associated with alum. Also, the decomposition of alum may occur at higher temperature. These two would reduce product yield.