Assertion :In the precipitation of the iron group in quantitative analysis, ammonium chloride is added before adding ammonium hydroxide to decrease a concentration of hydroxide ions. Reason: Addition of ammonium ions decreases the concentration of hydroxide ions in the concerned solution of above statement, following the law of ionic equilibrium.

Both Assertion and Reason are correct and Reason is the correct explanation for Assertion

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Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion

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Assertion is correct but Reason is incorrect

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Both Assertion and Reason are incorrect

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Solution

The correct option is A Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
An assertion is true: Quantitative analysis of Group (III) cations (iron group) is done by precipitating these cations as hydroxides by adding ammonium hydroxides in the presence of ammonium chloride.

$F e {C l}_{3} + 3 {N H}_{4} O H \to 3 {N H}_{4} C l + F e {(O H)}_{3} ↓ (r e d d i s h b r o w n p r e c i p i t a t e)$
$A l {C l}_{3} + 3 {N H}_{4} O H \to 3 {N H}_{4} C l + A l {(O H)}_{3} ↓ (w h i t e p r e c i p i t a t e)$

Reason is true and the correct explanation for Assertion: When ammonium hydroxide is used in the presence of ammonium chloride, the complete dissociation of ${N H}_{4} C l$ suppresses the dissociation of weak base ${N H}_{4} O H$ so that small concentration of ${O H}^{-}$ is sufficient to precipitate III group metals in the form of hydroxides as their solubility product is very less.

Hence, the answer is $A$ .

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