Some Basic Concepts of Chemistry Part 3

Concept:A primary standard is a highly pure, stable substance used to prepare a standard solution. It must meet specific criteria: purity, stability in air, non-hygroscopic, high solubility, high molar mass, and stoichiometric reaction.Explanation:Statement A: Some hydrated salts (e.g., Mohr’s salt, $\mathrm{FeSO_4 \cdot (NH_4)_2SO_4 \cdot 6H_2O}$) are stable enough to be used as primary standards. So A is correct.Statement B: A primary standard should not react with air components like $O_2$, $CO_2$, or moisture. So B is correct.Statement C: The reaction of a primary standard with the titrant must be fast and stoichiometric to give a sharp end point. So C is correct.Statement D: A primary standard must be readily soluble in water to prepare a solution. Thus D is incorrect.Statement E: A primary standard should have a high relative molar mass to reduce weighing error. Thus E is incorrect.Answer:The correct statements are A, B, and C only. Hence, the correct option is D (A, B and C Only).