Qualitative Analysis - Result Question 13

13. A sodium salt of an unknown anion when treated with $MgCl _2$ gives white precipitate only on boiling. The anion is

(2004,1 M)

(a) $SO _4^{2-}$

(b) $HCO _3^{-}$

(c) $CO _3^{2-}$

(d) $NO _3^{-}$

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Answer:

Correct Answer: 13. (b)

Solution:

A sodium salt of an unknown anion when treated with $MgCl$ gives white precipitate $\left(MgCO _3\right)$ only on boiling. Hence, the action must be $HCO _3^{-}$ion.

$MgCl _2+2 NaHCO _3 \xrightarrow{\Delta} MgCO _3+2 NaCl+H _2 O+CO _2$