Read the passage carefully and answer the questions.

Sodium dichromate $Na_2Cr_2O_7$ and Potassium dichromate $K_2Cr_2O_7$, are strong oxidizing agents; in acidic solution, its oxidising action can be represented as follows:

$Cr_2{O_7}^{2-}+ 14H^++ 6e^-→2Cr^{3+}+ 7H_2O$ $(E° = 1.33V)$

The chromates and dichromates are interconvertible in aqueous solution depending upon pH of the solution. The chromate ion is tetrahedral whereas the dichromate ion consists of two tetrahedra sharing one corner with Cr-O-Cr bond angle of 126°

Potassium permanganate is prepared by fusion of $MnO_2$ with an alkali metal hydroxide and an oxidising agent like $KNO_3$. This produces the dark green $K_2MnO_4$ which disproportionates in a neutral or acidic solution to give permanganate. It is a strong oxidizing agent. Hydrogen ion concentration of the solution plays an important part in the reduction of permanganate to manganate, manganese dioxide and manganese(II) salt.

Potassium permanganate forms dark purple (almost black) crystals which are isostructural with those of $KClO_4$. The salt is not very soluble in water (6.4 g/100 g of water at 293 K), but when heated it decomposes at 513 K.

Choose the correct statement from the options given below:

$K_2Cr_2O_7$ is used as primary standard in volumetric analysis.

The correct answer is Option (4) → $K_2Cr_2O_7$ is used as primary standard in volumetric analysis.

Core Concept

This question tests structures of oxyanions, acid–base equilibrium of chromate/dichromate, and properties of primary standards.

Option-wise Explanation

1. $\text{K}_2\text{MnO}_4$ is isostructural with $\text{KClO}_4$

$\text{KClO}_4$ contains the tetrahedral $\text{ClO}_4^-$ ion. $\text{K}_2\text{MnO}_4$ contains $\text{MnO}_4^{2-}$ (manganate), which is also tetrahedral, but the salts are not considered strictly isostructural in standard textbook context. The classic isostructural comparison is $\text{KMnO}_4$ with $\text{KClO}_4$, not $\text{K}_2\text{MnO}_4$. Hence this statement is taken as incorrect.

2. Chromate ion gets converted to a dichromate ion in basic medium

The equilibrium is:

$2\text{CrO}_4^{2-} + 2\text{H}^+ \rightleftharpoons \text{Cr}_2\text{O}_7^{2-} + \text{H}_2\text{O}$

Dichromate forms in acidic medium, while chromate is favored in basic medium. So this statement is wrong.

3. In $\text{K}_2\text{Cr}_2\text{O}_7$ all $\text{Cr}-\text{O}$ bonds have equal bond length

In dichromate ion, two tetrahedra share one oxygen. The bridging $\text{Cr}-\text{O}$ bond is longer than the terminal $\text{Cr}=\text{O}$ bonds. Therefore, all $\text{Cr}-\text{O}$ bonds are not equal.

4. $\text{K}_2\text{Cr}_2\text{O}_7$ is used as primary standard in volumetric analysis

Potassium dichromate is pure, stable, non-hygroscopic, and has high equivalent mass. Because of these properties, it is used as a primary standard oxidizing agent in redox titrations.