JEE Main 2025 Apr 3 Shift 2, Chemistry Q6: Principles Of Qualitative Analysis

JEE Main2025Apr 3, Shift 2Chemistry

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

A. Na $_{2}$ Cr $_{2}$ O $_{7}$

B. Oxalic acid

C. NaOH

D. FeSO $_{4} \cdot$ 6H $_{2}$ O

E. Sodium tetraborate

Choose the most appropriate answer from the options given below:

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

Solution

A primary standard must be highly pure, stable in air, non-hygroscopic, and have a well-defined composition so a known mass yields a known concentration.

Na $_{2}$ Cr $_{2}$ O $_{7}$ is hygroscopic, so its weighed mass is unreliable.

NaOH is strongly hygroscopic and also absorbs CO $_{2}$ from air, so its purity cannot be guaranteed by weighing.

FeSO $_{4} \cdot$ 6H $_{2}$ O is easily oxidised by atmospheric O $_{2}$ to Fe(III), so its Fe(II) content drifts.

Oxalic acid and sodium tetraborate (borax) are well-known primary standards.

$∴$ Compounds unfit as primary standards are A, C and D.

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JEE Main 2025 Apr 3 Shift 2, Chemistry Q6: Principles Of Qualitative Analysis

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