Chemical formula and colour of manganate ion is
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → MnO42−, Green
The manganate ion is MnO42− (Mn in +6 state); potassium manganate, \( \ce{K2MnO4} \), is dark green. It is formed when \( \ce{MnO2} \) is fused with KOH and an oxidising agent, and in neutral or acidic solution it disproportionates to purple permanganate.
\( \ce{2MnO2 + 4KOH + O2 -> 2K2MnO4 + 2H2O} \)
\( \ce{3MnO4^2- + 4H+ -> 2MnO4- + MnO2 + 2H2O} \)
Why the other options are incorrect
Option (2) MnO4−, Green — Incorrect. \( \ce{MnO4^{-}} \) is the permanganate ion, and it is purple, not green.
Option (3) MnO4−, Purple — Incorrect. The right colour for permanganate (Mn +7), but the question asks for manganate.
Option (4) MnO42−, Purple — Incorrect. The formula is right but manganate is green.
NCERT (2026-27 edition):
Oxides and Oxoanions of Metals – “… Potassium permanganate is prepared by fusion of MnO2 with an alkali metal hydroxide and an oxidising agent like KNO3. This produces the dark green K2MnO4 which disproportionates in a neutral or acidic solution to give permanganate.”