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:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → \(K_2Cr_2O_7\) is used as primary standard in volumetric analysis.
A primary standard is a substance whose standard solution can be made by weighing it directly: it must be available in a very pure form, be stable in air and not absorb moisture. Potassium dichromate meets these conditions (sodium dichromate is far more soluble and absorbs moisture), so a weighed amount of \( \ce{K2Cr2O7} \) gives a solution of known concentration for redox titrations.
Why the other options are incorrect
Option (1) \(K_2MnO_4\) is isostructural with \(KClO_4\) — Incorrect. The passage says this of potassium permanganate, \( \ce{KMnO4} \): its tetrahedral \( \ce{MnO4^-} \) ion and single \( \ce{K+} \) match the \( \ce{ClO4^-} \) and \( \ce{K+} \) of \( \ce{KClO4} \). Potassium manganate, \( \ce{K2MnO4} \), has two \( \ce{K+} \) ions for each anion, so its crystal cannot be built like that of \( \ce{KClO4} \).
Option (2) chromate ion gets converted to a dichromate ion in basic medium. — Incorrect. It is the other way round. Chromate changes to dichromate in acidic medium, and dichromate goes back to chromate in basic medium:
\( \displaystyle \ce{2CrO4^2- + 2H^+ -> Cr2O7^2- + H2O} \)
\( \displaystyle \ce{Cr2O7^2- + 2OH^- -> 2CrO4^2- + H2O} \)
Option (3) in \(K_2Cr_2O_7\) all \(Cr-O\) bonds have equal bond length. — Incorrect. The dichromate ion is two \( \ce{CrO4} \) tetrahedra sharing one corner. It has six terminal Cr–O bonds and two bridging Cr–O bonds in the Cr–O–Cr link; the bridging bonds are longer, so the bond lengths are not all equal.
NCERT (2026-27 edition):
Oxides and Oxoanions of Metals – …
