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Question
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, l, Zn, Pb, Fe)
(a) Which metal salt solution was used?
(b) What is the formula of white gelatinous precipitate obtained?
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Solution
(a) Zinc (Zn ) metal salt solution was used.
(b) The formula of white gelatinous precipitate is Zn(OH)2.
Concept Insight :
\[\ce{\underset{\text{Colourless}}{ZnSO4} + 2NH4OH ->\underset{\text{White ppt.}}{Zn(OH)2} + (NH4)2SO4}\]
With excess of NH4OH the oreciojtate of zinc(II) hydroxide dissolves as:
\[\ce{Zn(OH)2 +\underset{\text{In excess}}{(NH4)2SO4 + 2NH4OH} ->\underset{\text{Tetrammine zinc(II)Sulphate(Colourless solution)}}{[Zn(NH3)4]SO4}+ 4H2O}\]
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