Solved: CHEMISTRY Question for NEET

NEET CHEMISTRYMedium

Identify the correct order of solubility in aqueous medium:

ZnS > Na₂S > CuS

Na₂S > CuS > ZnS

Na₂S > ZnS > CuS

CuS > ZnS > Na₂S

Na₂S (Sodium Sulfide): Salts of alkali metals (Group 1) are generally soluble in water. Therefore, $Na_2S$ has the highest solubility.
ZnS vs CuS: Transition metal sulfides are generally sparingly soluble. The solubility is determined by their Solubility Product constants ( $K_{sp}$ ).

CuS: Precipitates in Group II of qualitative analysis in the presence of dilute HCl (acidic medium). This indicates a very low $K_{sp}$ (\approx $10^{-36}$ ), meaning it is highly insoluble [Source 235 mentions CuS has 'such a low solubility'].
ZnS: Precipitates in Group IV of qualitative analysis only in the presence of $NH_4OH$ (alkaline medium). It requires a higher concentration of sulfide ions to precipitate, indicating a comparatively higher $K_{sp}$ (\approx $10^{-24}$ ) than CuS [Source 235 confirms ZnS precipitates in alkaline solution].

Order: Soluble ( $Na_2S$ ) > Sparingly Soluble ( $ZnS$ ) > Highly Insoluble ( $CuS$ ).

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