Question

Potash alum is a double salt having the chemical formula${\mathrm{K}}_2{\mathrm{SO}}_4.{\mathrm{Al}}_2{\left({\mathrm{SO}}_4\right)}_3.24{\mathrm{H}}_2\mathrm{O}.$ It dissolves in water to form a clear solution.

Between what pH range will the clear solution lie? Explain.

Answer 1

• Double salt is an ionic compound comprised of two or more simple salts.
• Potash alum is a double salt that is made from 2 normal salts Potassium sulfate $\left({\mathrm{K}}_2{\mathrm{SO}}_4\right)$ and Aluminium sulfate $\left({\mathrm{Al}}_2{\left({\mathrm{SO}}_4\right)}_3\right)$.
• When a clear solution of Potash alum is treated with blue litmus paper, it turns red, which means it is acidic in nature.
• Potash alum when hydrolyzed, $\left({\mathrm{Al}}_2{\left({\mathrm{SO}}_4\right)}_3\right)$ is dissociates into ${\mathrm{Al}}^{3+}$ ions and ${{\mathrm{SO}}_4}^{2-}$ ions.
• Aluminum sulfate is a salt of a strong acid i.e., Sulfuric acid $\left({\mathrm{H}}_2{\mathrm{SO}}_4\right)$ and a weak base i.e., Aluminium hydroxide $\left(\mathrm{Al}{\left(\mathrm{OH}\right)}_3\right)$, hence the solution has the pH value less than 7, which means it is acidic.