Q20 / 75 · JEE Main 2025 (3 Apr Shift 2) · Chemistry

Compounds that should not be used as primary standards in titrimetric analysis are :
A. $\mathrm{Na}_2 \mathrm{Cr}_2 \mathrm{O}_7$
B. Oxalic acid
C. NaOH
D. $\mathrm{FeSO}_4 \cdot 6 \mathrm{H}_2 \mathrm{O}$
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
  • Correct answer
    D
    A, C and D Only

Solution

The primary standard is a highly pure stable compound with a known exact composition that can be accurately weighed and dissolved to creat a solution of known concentration.
NaOH is hygroscopic and can't be used.
$\mathrm{FeSO}_4 \cdot 6 \mathrm{H}_2 \mathrm{O}$ is unstable and can be easily oxidised.
$\mathrm{Na}_2 \mathrm{Cr}_2 \mathrm{O}_7$ is hygroscopic and can't be used.

Hence, the correct answer is option: 'D'.

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