sblock Elements 1 Question 27
27. A sodium salt of an unknown anion when treated with $\mathrm{MgCl}_2$ gives white precipitate only on boiling. The anion is
(2004, 1M)
(a) $\mathrm{SO}_4^{2-}$
(b) $\mathrm{HCO}_3^{-}$
(c) $\mathrm{CO}_3^{2-}$
(d) $\mathrm{NO}_3^{-}$
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Answer:
Correct Answer: 27. (b)
Solution: $\mathrm{Mg}\left(\mathrm{HCO}_3\right)_2$ on boiling decomposes to give white precipitate of $\mathrm{MgCO}_3$ as:
$$ \mathrm{Mg}\left(\mathrm{HCO}_3\right)_2(\mathrm{aq}) \xrightarrow{\text { Heat }} \mathrm{MgCO}_3 \downarrow+\mathrm{H}_2 \mathrm{O}+\mathrm{CO}_2 \uparrow $$
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