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NEET CHEMISTRYThe d-and f-Block ElementsMedium

Question

Identify the correct order of solubility in aqueous medium:

A

ZnS > Na₂S > CuS

B

Na₂S > CuS > ZnS

C

Na₂S > ZnS > CuS

D

CuS > ZnS > Na₂S

Step-by-Step Solution

  1. Na₂S (Sodium Sulfide): Salts of alkali metals (Group 1) are generally soluble in water. Therefore, Na2SNa_2S has the highest solubility.
  2. ZnS vs CuS: Transition metal sulfides are generally sparingly soluble. The solubility is determined by their Solubility Product constants (KspK_{sp}).
  • CuS: Precipitates in Group II of qualitative analysis in the presence of dilute HCl (acidic medium). This indicates a very low KspK_{sp} (\approx 10−3610^{-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 NH4OHNH_4OH (alkaline medium). It requires a higher concentration of sulfide ions to precipitate, indicating a comparatively higher KspK_{sp} (\approx 10−2410^{-24}) than CuS [Source 235 confirms ZnS precipitates in alkaline solution].
  1. Order: Soluble (Na2SNa_2S) > Sparingly Soluble (ZnSZnS) > Highly Insoluble (CuSCuS).

Exam Context & Concepts Covered

This question aligns with the NEET CHEMISTRY syllabus, specifically targeting concepts from The d-and f-Block Elements. Mastering this topic is crucial for scoring well in the upcoming medical entrance examinations. Solving conceptually related problems will help you understand the nuances of these concepts and improve your problem-solving speed.

CHEMISTRYThe d-and f-Block Elementsidentifycorrectsolubilityaqueousmedium

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