A coloured solution known to contain two metal ions, was treated with excess cold sodium hydroxide solution. When filtered a whitish solid, slowly changing to brown, was retained on the filter paper a

Question

A coloured solution known to contain two metal ions, was treated with excess cold sodium hydroxide solution. When filtered a whitish solid, slowly changing to brown, was retained on the filter paper and a colourless solution collected as the filtrate. Dropwise addition of hydrochloric acid to the filtrate produced a white ppt. which dissolved in excess acid. Treatment of the residue on filter paper with a solution of strong oxidiser produced a reddish-violet solution. Indicate any pairs of ions:

Prashant Kotian · Accepted Answer

Correct answer: A. Step 1: Ammonium chloride (NH4Cl) is a strong electrolyte that dissociates completely to provide a high concentration of NH4+ ions. Step 2: The high concentration of NH4+ suppresses the dissociation of (NH4)2CO3 through the common ion effect, keeping the carbonate ion (CO3^2-) concentration low. Step 3: This low concentration of CO3^2- is sufficient to exceed the solubility products of Group V carbonates (BaCO3, SrCO3, CaCO3) but is too low to precipitate MgCO3. Therefore, MgCO3 is not precipitated in the presence of NH4Cl, matching option (b).

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