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ICSE Class 10 Chemistry Question 1 of 10

Analytical Chemistry — Question 4

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Question 4

On adding dilute ammonia solution to a colourless solution of a salt, a white gelatinous precipitate appears. This precipitate however dissolves on addition of excess of ammonia solution.

(a) From the following list, identify which metal salt solution was used above?
    Na, Al, Zn, Pb, Fe

(b) What is the formula of the white gelatinous precipitate obtained?

(c) Give the balanced equation(s) when sulphate of this metal reacts with ammonia solution in excess.

Answer

(a) Zinc (Zn) metal salt solution was used

(b) Zn(OH)2

(c) The balanced equations are given below:

ZnSO4colourless solution+2NH4OH⟶Zn(OH)2↓white gelatinous ppt+(NH4)2SO4colourless in solution\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 of NH4OH ppt. dissolves

Zn(OH)2+(NH4)2SO4+2NH4OH⟶[(Zn(NH3)4]SO4Tetraamminezinc (II) sulphate+4H2O\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}