$K_2Cr_2O_7 + H_2O_2 \xrightarrow{H^{+}} $

The oxoanion in which the oxidation state of the central atom is same as its group number in the periodic table is

The yellow colour solution of $Na_2CrO_4$ changes to orange-red on passing $CO_2$ gas due to the formation of $:-$

The fusion of chromite ore $(FeCr_{2}O_{4})$ with $Na_{2}CO_{3}$ in air gives a yellow solution upon addition of water. Subsequent treatment with $H_{2}SO_{4}$ produces an orange solution. The yellow and orange colours,respectively,are due to the formation of

The colorless species is

One of the important uses of ferrous sulphate is in the