Model Paper 1 — Question 21

(a) When ammonium hydroxide (NH₄OH) is added to zinc nitrate solution [Zn(NO₃)₂], a gelatinous white ppt of zinc hydroxide [Zn(OH)₂] is obtained which is soluble in excess of NH₄OH.

$\underset{\text{colourless solution}}{{\text{ZnSO}_4}} + 2\text{NH}_4\text{OH} \longrightarrow \underset{\text{white gelatinous ppt}}{\text{Zn(OH)}_2↓} + \underset{\text{colourless in solution}}{(\text{NH}_4)_2\text{SO}_4}$

(With excess NH₄OH ppt. dissolves)

$\text{Zn(OH)}_2 + (\text{NH}_4)_2\text{SO}_4 + 2\text{NH}_4\text{OH} \longrightarrow \underset{\underset{\text{zinc (II) sulphate}}{\text{Tetraammine}}}{[(\text{Zn(NH}_3)_4]\text{SO}_4} + 4\text{H}_2\text{O}$

On the other hand, calcium nitrate solution [Ca(NO₃)₂] does not give any ppt. even when excess of ammonium hydroxide is added.

(b) When dil. sulphuric acid is added to sodium carbonate and heated, colourless, odourless gas is evolved which turns lime water milky and has no effect on KMnO₄ or K₂Cr₂O₇ solutions.

Na₂CO₃ + 2HCl ⟶ 2NaCl + H₂O + CO₂ ↑

When dil. sulphuric acid is added to sodium sulphite and heated, colourless gas with suffocating odour is evolved which turns lime water milky. It turns acidified K₂Cr₂O₇ from orange to clear green and pink coloured KMnO₄ to clear colourless.

Na₂SO₃ + 2HCl ⟶ 2NaCl + H₂O + SO₂ ↑

Hence, the two compounds can be distinguished.