Which one of the following species is stable in aqueous solution?

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

Consider the following reactions:
$K_2Cr_2O_7 + NaCl + H_2SO_4 \rightarrow CrO_2Cl_2 + \text{Side Products}$
$CrO_2Cl_{2(Vapour)} + NaOH \rightarrow B + NaCl + H_2O$
$B + H^+ \rightarrow C + H_2O$
The number of terminal $'O'$ atoms present in the compound $'C'$ is $..........$

Describe the preparation of potassium permanganate. How does the acidified permanganate solution react with $(i)$ iron $(II)$ ions,$(ii)$ $SO_2$,and $(iii)$ oxalic acid? Write the ionic equations for the reactions.

The fusion of manganese dioxide with $KOH$ in the presence of an oxidizing agent like $KNO_3$ yields a compound. Identify the compound and its color.

Complete the following reactions:
$(a)$ $4FeCr_2O_4 + 8Na_2CO_3 + 7O_2 \to 8Na_2CrO_4 + 2Fe_2O_3 + 8CO_2$
$(b)$ $2Na_2CrO_4 + 2H^+ \to Na_2Cr_2O_7 + 2Na^+ + H_2O$
Identify the species $X$ in both reactions.