Question

Identify the incorrect statements regarding primary standard of titrimetric analysis
(A) It should be purely available in dry form.
(B) It should not undergo chemical change in air.
(C) It should be hygroscopic and should react with another chemical instantaneously and stoichiometrically.
(D) It should be readily soluble in water.
(E) KMnO4 & NaOH can be used as primary standard.
Choose the correct answer from the options given below :

• (C) and (D) only
• (B) and (E) only
• (A) and (B) only
• (C) and (E) only

Answer 1

The Correct Option is D

To determine the incorrect statements regarding the primary standard of titrimetric analysis, let's evaluate each option:

1. (A) It should be purely available in dry form: This is a correct statement. Primary standards should be of a high purity and stable form, typically in a dry state, to ensure accurate measurement.
2. (B) It should not undergo chemical change in air: This is also a correct statement. Primary standards must be stable in air so they don't react with moisture or gases, ensuring that their mass remains constant over time.
3. (C) It should be hygroscopic and should react with another chemical instantaneously and stoichiometrically: This statement is incorrect. A primary standard should not be hygroscopic (meaning it should not absorb moisture from the air) as this would affect its mass and thus the accuracy of the titration.
4. (D) It should be readily soluble in water: This is a correct statement. Solubility ensures that the compound can dissolve fully to provide a consistent and accurate reaction.
5. (E) KMnO₄ & NaOH can be used as primary standards: This statement is incorrect. KMnO₄ is not used as a primary standard due to its propensity to decompose and its instability in air. Similarly, NaOH is hygroscopic and absorbs water and carbon dioxide from the air, making it unsuitable as a primary standard.

Based on this analysis, the incorrect statements are (C) and (E).

Answer 2

A primary standard substance used in titrimetric analysis must satisfy the following criteria:

1. It should be available in pure and dry form.
2. It must not be hygroscopic (i.e., it should not absorb moisture) or undergo any chemical change in air.
3. It should be readily soluble in water.
4. KMnO₄ and NaOH are not suitable as primary standards due to their hygroscopic nature and tendency to absorb CO₂ from air, making them impure.

Incorrect Statements:

• (C): The substance should not be hygroscopic. Hence, this statement is incorrect.
• (E): Both KMnO₄ and NaOH are unsuitable as primary standards. This statement is also incorrect.

Final Answer is option (4).