Statement A is correct. Potassium dichromate acts as a strong oxidising agent in acidic medium and oxidises Fe2+ to Fe3+, converting FeSO4 to Fe2(SO4)3.
Statement B is incorrect. Sodium dichromate is deliquescent (absorbs moisture from the atmosphere), which prevents it from being weighed accurately. Therefore, it cannot be used as a primary standard in volumetric estimation. Potassium dichromate is used instead.
Statement C is correct. Chromate (CrO42−) and dichromate (Cr2O72−) ions are interconvertible in aqueous solution depending on the pH of the solution. In acidic medium, chromate changes to dichromate, and in basic medium, dichromate changes to chromate: 2CrO42−+2H+⇌Cr2O72−+H2O.
Statement D is correct. The structure of the dichromate ion consists of two tetrahedral CrO3 groups joined by a bridging oxygen atom, and the Cr−O−Cr bond angle is 126∘.
Therefore, statements A, C, and D are correct.
Answer: A, C and D Only