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Analytical Chemistry — Chemistry STD 10 — Question

ICSE BoardEnglish MediumSTD 10ChemistryAnalytical Chemistry3 Marks
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?
✓

Answer

(a) Zinc ( $Zn )$ metal salt solution was used.
(b) The formula of white gelatinous precipitate is $Zn ( OH )_2$.
Concept Insight :
$
\underset{\text { Colourless }}{ ZnSO _4}+2 NH _4 OH \longrightarrow \underset{\text { White ppt. }}{ Zn ( OH )_2}+\left( NH _4\right)_2 SO _4
$
With excess of $NH _4 OH$ the oreciojtate of zinc(II) hydroxide dissolves as:
$
Zn ( OH )_2+\underset{\text { In excess }}{\left( NH _4\right)_2 SO _4+2 NH _4 OH } \longrightarrow \underset{\text { Tetrammine xinc(II)Sulphate(Colourless solution) }}{\left[ Zn \left( NH _3\right)_4\right] SO _4}+4 H _2 O
$

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