Question

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 another 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 the options given below:

• A. A, B and C Only
• B. A, B, C and E Only
• C. A, B and E Only
• D. D and E Only

Hint:

A good primary standard must be pure, stable in air, soluble in water, and react stoichiometrically with the titrant.

Answer 1

The Correct Option is C

To determine the correct statements regarding primary standards, we need to understand the properties of primary standards. A primary standard is a highly pure compound used in titration to compare the concentrations of solutions.

1. Statement A: Hydrated salts can be used as a primary standard.

   This statement is generally true. Hydrated salts can be used as primary standards if they have a stable and known composition. For example, sodium carbonate (Na₂CO₃) is often used as a primary standard in its decahydrate form, provided it does not lose water to the air.

2. Statement B: Primary standards should not undergo any reaction with air.

   This statement is true. A primary standard should be stable and should not react with the atmosphere; otherwise, its mass will change, leading to inaccuracies. Thus, it should not absorb moisture from the air or react with gases like carbon dioxide.

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

   This statement is true. In titration, the reaction between the primary standard and the known substance should be rapid and follow a clear stoichiometric relationship to ensure precise calculations.

4. Statement D: Primary standard should not be soluble in water.

   This statement is false. In fact, a primary standard should be sufficiently soluble in solvent (often water) for preparation of solutions. Insolubility would make it impractical for use in titration.

5. Statement E: Primary standard should have low relative molar mass.

   This statement is true. A low relative molar mass implies that less mass is needed to achieve a mole, which can help minimize weighing errors, leading to more accurate measurements.

Based on the analysis, the correct statements are A, B, and E. Hence, the correct answer is A, B, and E Only.