JEE Chemistry 2025 Question with Solution

Given: The compounds listed are Na$_2$Cr$_2$O$_7$, Oxalic acid, NaOH, FeSO$_4$.6H$_2$O, and Sodium tetraborate.

Find: Which compounds should not be used as primary standards in titrimetric analysis.

A primary standard in titrimetric analysis should have high purity, known composition, stability, should be non-hygroscopic, and should not react with air easily.

From the given analysis:

1. Na$_2$Cr$_2$O$_7$ is not preferred as a primary standard.
2. Oxalic acid is used as a primary standard.
3. NaOH is hygroscopic and absorbs moisture and $\text{CO}_2$ from air, so it is unsuitable.
4. FeSO$_4$.6H$_2$O is unstable and gets oxidized in air, so it is unsuitable.
5. Sodium tetraborate is a good primary standard.

Therefore, the compounds that should not be used as primary standards are A, C and D.

The correct option is D.

Given: A primary standard must be a highly pure, stable compound with definite composition.

Find: Identify which listed compounds fail the criteria for a primary standard.

The important criteria are:

1. High purity
2. Stability
3. Non-hygroscopic nature
4. No reaction with air
5. Known stoichiometric composition

Now evaluate each compound:

1. Na$_2$Cr$_2$O$_7$: It is considered unsuitable here.
2. Oxalic acid: Stable and used as a primary standard.
3. NaOH: Hygroscopic and absorbs moisture and carbon dioxide from air, so accurate weighing is not possible.
4. FeSO$_4$.6H$_2$O: Unstable and easily oxidized by atmospheric oxygen.
5. Sodium tetraborate: Used as a primary standard.

Hence, the compounds not used as primary standards are Na$_2$Cr$_2$O$_7$, NaOH, and FeSO$_4$.6H$_2$O.

Thus, the correct answer is A, C and D Only, which corresponds to option D.