Question

A double salt which contains fourth-period alkali metal (A) on heating at 500 K gives (B). aqueous solution of (B) gives white precipitate with BaCl₂ and gives a red colour compound with alizarin. Identify A and B.

Answer 1

1. A double salt which contains fourth-period alkali metal (A) is potash alum

\[\ce{K2SO4.Al2(SO4)3.24H2O}\]

2. On heating potash alum (A) 500 k give anhydrous potash alum (or) burnt alum (B).

\[\ce{\underset{\text{[Potash alum (A)]}}{K2SO4.Al2(SO4)3.24H2O} ->[500 K] \underset{\text{[Burnt alum (B)]}}{K2SO4.Al2(SO4)3} + 24H2O}\]

3. Aqueous solution of burnt alum, has sulphates ion, potassium ion and aluminium ion. Sulphate ion reacts with BaCl₂ to form a white precipitate of Barium Sulphate

\[\ce{(SO4)2 + BaCl2 -> BaSO4 + 2Cl^-}\]

Aluminium ion reacts with alizarin solution to give a red colour compound.