Question

Aqueous solution of potash alum is
(A) Acidic
(B) Basic
(C) Neutral
(D) None of the above

Answer 1

Hint: As potash alum is a salt of potassium sulphate and aluminium sulphate, on hydrolysis, its aqueous solution will have dissociated ions such as \[A{l^{3 + }}\], \[{K^ + }\] and \[SO_4^{2 - }\] along with water molecules. Aluminium has high positive charge density and can easily pull the hydroxide ions towards it, leaving behind the protons.

Complete step-by-step solution:
Potash alum is a name given to a chemical compound, potassium aluminium sulphate having chemical formula \[{K_2}S{O_4}.A{l_2}{(S{O_4})_3}.24{H_2}O\].
Alum stands for the series of double salts that contain sulphate ions. When alum is crystallised from a concentrated equimolar solution of ammonium sulphate and potassium sulphate. It is popularly known as ‘fitkari’, used as medicine, water treatment and in the production of pigments or dye.
The chemical reaction involved can be written as –
\[{K_2}S{O_4}(aq) + A{l_2}{(S{O_4})_3}(aq) \to {K_2}S{O_4}.A{l_2}{(S{O_4})_3}.24{H_2}O\]
Ammonium sulphate and potassium sulphate are the two inorganic salts when mixed in a definite proportion yield a double salt called potash alum on crystallisation with water. When tested with litmus paper, it turns blue litmus paper red, confirming its acidic nature. This is because when the potash alum is hydrolysed, \[A{l_2}{(S{O_4})_3}\] dissociates into ions as it is a salt of weak base of aluminium hydroxide and sulphuric acid as strong acid. \[A{l^{3 + }}\]ions are solvated by water molecules due to its high positive charge density and produce excess \[{H^ + }\] ions. This tells us that the solution has become acidic.
\[A{l^{3 + }} + 3{H_2}O \to Al{(OH)_3} + 3{H^ + }\]

Hence, the correct option is (A).

Additional information: it is used as a fire-retardant, leather tanning substance and for casting of steel parts. It can also be prepared by leaching alumina from aluminium ore, Bauxite.

Note: When aluminium sulphate dissociates into ions on hydrolysis, along with aluminium ion, it forms aluminium hydroxide as well. But since \[Al{(OH)_3}\] is a weak base, it doesn’t ionise completely and results in partial hydrolysis of aluminium ions.