Question

Solution A is a strong acid.

Solution B is a weak acid.

Solution C is a strong alkali.

1. Which solution contains solute molecules in addition to water molecules?
2. Which solution will give a gelatinous white precipitate with zinc sulphate solution? The precipitate disappears when an excess of the solution is added.
3. Which solution could be a solution of glacial acetic acid?
4. Give an example of a solution which is a weak alkali.

Answer 1

1. Solution B contains solute molecules in addition to water molecules. A weak acid is an acid which dissociates only partially in aqueous solution, thereby producing a low concentration of hydrogen [H⁺] ions [or H₃O⁺ ions]. 
   Example: \[\ce{CH3COOH <=> CH3COO- + H+}\] [contains molecules and ions]
2. Solution C will give a gelatinous white precipitate with zinc sulphate solution. Strong alkalis react with certain salt solutions to precipitate insoluble hydroxide. Hence, \[\ce{ZnSO4 + 2NaOH -> Na2SO4 + Zn(OH)2}\] [gelatinous white precipitate]
3. Solution B, a weak acid, could be a solution of glacial acetic acid. Anhydrous acetic acid, on cooling, forms crystals of glacial acetic acid, and glacial acetic acid is a weak acid.
4. Ammonium hydroxide (NH₄OH) is a weak alkali.