Compounds that should not be used as primary standards in titrimetric analysis are : A. Na2Cr2O7 B. Oxalic acid C. NaOH D. FeSO4 . 6H2O E. Sodium tetraborate Choose the most appropriate answer from the options given below:

The correct answer is Option 1. Full mathematical solution steps and hints are accessible in the interactive StemCET workspace.

Chemistry Q63 – JEE-MAIN 2025

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Home Chemistry Basic ChemistryQuestion #60

Compounds that should not be used as primary standards in titrimetric analysis are : A.

N a_{2} C r_{2} O_{7}

B. Oxalic acid C.

N a O H

F e S O_{4} \cdot 6 H_{2} O

E. Sodium tetraborate Choose the most appropriate answer from the options given below:

Clue 1 of 3: Primary Standards

Recall the key characteristics of a primary standard, such as high purity, stability, non-hygroscopic nature, and high equivalent weight.

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Ninja Strategy•Identify Definite Non-Standards First

First, identify the compounds that are absolutely not primary standards ( $N a O H$ and $F e S O_{4} \cdot 6 H_{2} O$ ). Then, eliminate options that do not include both of these. Finally, choose the most plausible option among the remaining, considering subtle distinctions in primary standard definitions.

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Step 1: Define Primary Standards✦ Active

A primary standard is a substance of high purity, stability, non-hygroscopic nature, and high equivalent weight, used to prepare standard solutions. Compounds that do not meet these criteria are unsuitable as primary standards.

Step 2: Evaluate Each Compound's Suitability as a Primary Standard○ Expand

Let's evaluate each compound given: A. $N a_{2} C r_{2} O_{7}$ (Sodium dichromate): While often considered a primary standard, some sources prefer $K_{2} C r_{2} O_{7}$ due to $N a_{2} C r_{2} O_{7}$ 's potential hygroscopicity, making it less ideal in very strict definitions. Thus, it can be considered 'not a primary standard' in some contexts. B. Oxalic acid ( $H_{2} C_{2} O_{4} \cdot 2 H_{2} O$ ): A well-known and widely accepted primary standard. C. $N a O H$ (Sodium hydroxide): Definitely not a primary standard due to its highly hygroscopic nature and its tendency to absorb $C O_{2}$ from the air, leading to variable composition. D. $F e S O_{4} \cdot 6 H_{2} O$ (Ferrous sulfate hexahydrate): Not a primary standard as ferrous salts are easily oxidized by air, making them unstable. E. Sodium tetraborate ( $N a_{2} B_{4} O_{7} \cdot 10 H_{2} O$ , Borax): A well-known and widely accepted primary standard for acid-base titrations.

Step 3: Identify Non-Primary Standards and Select the Most Appropriate Option○ Expand

Based on the evaluation, C ( $N a O H$ ) and D ( $F e S O_{4} \cdot 6 H_{2} O$ ) are unequivocally not primary standards. Considering the potential hygroscopicity of $N a_{2} C r_{2} O_{7}$ (A) as a reason to classify it as 'not a primary standard' in a stricter sense, the set of compounds that should not be used as primary standards would be A, C, and D. This set matches Option 1.

💡 Teacher's Secret Hint

In cases where the ideal option is not present, choose the 'most appropriate' answer by identifying the definite non-standards and then considering which of the generally accepted primary standards might have minor drawbacks that could lead to their exclusion in a strict interpretation.

✦ STEM Console utilizes AI models to generate step-by-step explanations and math clues. AI can make mistakes.

Chemistry Question 63 – JEE-MAIN 2025

Clue 1 of 3: Primary Standards

🧠 Full Solution Path