Some of the following reagents are used as primary standard
I. `KMnO_(4)`
II. NaOH
III . `K_(2)Cr_(2)O_(7)`
IV. `FeSO_(4)(NH_(4))_(2)SO_(4).6H_(2)O`
V. `H_(2)C_(2)O_(4).2H_(2)O`
Select the primary standard

Options

A. II,IV
B. I,II
C. I,III, V
D. All of these

Answer

To determine which of the given reagents are primary standards, we need to understand the definition and characteristics of a primary standard. A primary standard is a reagent that is:

1. Highly pure.
2. Stable in air.
3. Non-hygroscopic (does not absorb moisture from the air).
4. Has a known and consistent stoichiometry.
5. Can be accurately weighed.

Now, let's evaluate each of the given reagents:

1. **KMnO₄ (Potassium permanganate)**:
- It is a strong oxidizing agent and can be used as a primary standard. However, it is not stable in the presence of light and can decompose, which makes it less ideal as a primary standard. But it is often used in titrations after being standardized with a primary standard.

2. **NaOH (Sodium hydroxide)**:
- NaOH is hygroscopic and absorbs moisture from the air, which makes it unsuitable as a primary standard. It is often standardized against a primary standard.

3. **K₂Cr₂O₇ (Potassium dichromate)**:
- This compound is a stable, pure, and non-hygroscopic reagent. It can be accurately weighed and is commonly used as a primary standard in redox titrations.

4. **FeSO₄(NH₄)₂SO₄·6H₂O (Mohr's salt)**:
- Mohr's salt is not a primary standard because it is not stable in air and can oxidize, which affects its purity and weight consistency.

5. **H₂C₂O₄·2H₂O (Oxalic acid dihydrate)**:
- Oxalic acid is a stable and pure compound that can be accurately weighed. It is often used as a primary standard in acid-base titrations.

Based on this evaluation, the primary standards from the given list are:

- KMnO₄ (but with caution due to stability issues)
- K₂Cr₂O₇
- H₂C₂O₄·2H₂O

Thus, the correct answer is: **I, III, and V**.

### Final Answer:
The primary standards among the given reagents are:
- I. KMnO₄
- III. K₂Cr₂O₇
- V. H₂C₂O₄·2H₂O