Identify the correct statements
A. Hydrated salts can be used as primary standard.
B. Primary standard should not undergo any reaction with air.
C. Reactions of primary standard with an other substance should be instantaneous and stoichiometric.
D. Primary standard should not be soluble in water.
E. Primary standard should have low relative molar mass.
Choose the correct answer from th e options given below :

We need to identify correct statements about primary standards.

A. Hydrated salts can be used as primary standard.

True. Hydrated salts with stable and definite water of crystallisation (e.g., oxalic acid dihydrate $$H_2C_2O_4 \cdot 2H_2O$$, Mohr's salt $$(NH_4)_2Fe(SO_4)_2 \cdot 6H_2O$$) can serve as primary standards because their composition is well-defined and reproducible.

B. Primary standard should not undergo any reaction with air.

True. A primary standard must be stable in air - it should not be hygroscopic (absorb moisture), should not react with $$CO_2$$ or $$O_2$$ in air. Otherwise its composition would change, making accurate weighing impossible.

C. Reactions of primary standard with another substance should be instantaneous and stoichiometric.

True. The reaction must be quantitative (complete) and follow a definite stoichiometry so that the equivalence point can be accurately determined.

D. Primary standard should not be soluble in water.

False. On the contrary, a primary standard should be readily soluble in water so that accurate solutions of known concentration can be prepared.

E. Primary standard should have low relative molar mass.

False. A primary standard should ideally have a high molar mass to minimize weighing errors (a larger mass for a given number of moles means the percentage error in weighing is smaller).

The correct answer is Option 2: A, B and C only.