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Questions
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. Identify (Choose from Na, Al, Zn, Pb, Fe).
- Which metal salt solution was used?
- What is the formula of the white gelatinous precipitate obtained?
On addition of dilute ammonia solution to a colourless salt solution, a white gelatinous precipitate is formed which dissolves when ammonia solution is in excess. Identify the metal from Na, Al, Pb, Fe, Zn whose salt solution is used. What is the formula of the precipitate formed?
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Solution
(a) Zinc (Zn) metal salt solution was used.
(b) The formula of the white gelatinous precipitate is Zn(OH)2.
Concept Insight:
\[\ce{\underset{Colourless}{ZnSO4} + 2NH4OH ->\underset{White ppt.}{Zn(OH)2} + (NH4)2SO4}\]
With an excess of NH4OH the precipitate of zinc(II) hydroxide dissolves as:
\[\ce{Zn(OH)2 +\underset{In excess}{(NH4)2SO4 + 2NH4OH} ->\underset{\underset{(Colourless solution)}{Tetrammine zinc(II)Sulphate}}{[Zn(NH3)4]SO4}+ 4H2O}\]
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