You are provided with PbCO_3, ZnCO_3 and CaCO_3. How will you ide… - Vidyadip

Question

You are provided with $PbCO_3, ZnCO_3$ and $CaCO_3$. How will you identify these cations?

✓

Answer

By use of Ammonium hydroxide we can identify the ions $PbCO _3, ZnCO _3$ and $CaCO _3$ as:
(i)
$
PbCO _3+2 NH _4 OH \longrightarrow Pb ( OH )_2 \downarrow+2 NH _4 NO _3
$
$Ob ( OH )_2$ forms white precipitate which are insoluble in excess ammonium hydroxide.
ii)
$
ZnCO _3+ NH _4 OH \longrightarrow Zn ( OH )_2 \downarrow+2 Na _2 CO _3
$
$Zn ( OH )_2$ forms white gelatinous precipitate which are soluble in excess ammonium hydroxide. iii)
$
CaCO _3+ NH _4 OH \longrightarrow Ca ( OH )_2+2 Na _2 CO _3
$
No precipitation of $Ca ( OH )_2$ occurs even with addition of excess of $NH _4 OH$. Because the concentration of hydroxide ion from ammonium hydroxide is so low that it cannot precipitate the hydroxide of calcium.
Concept Insight : Some precipitated metallic hydroxides by ammonium hydroxide become soluble aydroxides when treated with excess of ammonium hydroxide due to the formation of a soluble complex salt in the presence of excess of ammonium hydroxide.

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