Geo-Scientist Prelims 2020 Chemistry Q37: Which one of the following is a primary standard substance?

Answer: (b) K₂Cr₂O₇

A primary standard must be pure, stable, non-hygroscopic, of known formula and high molar mass. K₂Cr₂O₇ fits → (b). KMnO₄ (decomposes, MnO₂ impurity), Na₂S₂O₃ (efflorescent, bacterial decay), NaOH (hygroscopic, absorbs CO₂) are secondary standards.