Explain by giving suitable reason.
(a). Yellow coloured aqueous solution of sodium chromate changes to orange-red when `CO_2` under pressure is passed.
(b). Green solution of potassium manganate, `K_2MnO_4`, turns purple when `CO_2` is circulated.
(c). `Hg^(2+)` and `Hg_2^(2+)` salts are colourless.
(d). `Cu^(2+)` salts are paramagnetic while `Cu^(o+)` salts are diamagnetic in nature.

Explanation

(a). `CO_2` gives `H^(o+)` ions which are responsible for the conversion of chromate into dichromate (orange- red).
`CO_2+H_2Ounderset(larr)rarrHCO_3^(ɵ)+H^(o+)`
`2CrO_4^(2-)+2H^(o+)toCr_2O_7^(2-)+H_2O`
(b). `CO_2` gives `H^(o+)` ions which are responsible for conversion of `MnO_4^(2-)` into `MnO_4^(ɵ)` purple
`CO_2+H_2Ounderset(larr)rarrH^(o+)+HCO_3^(ɵ)`
`3MnO_4^(2-)+4H^(o+)tounderset(purpl e)(2MnO_4^(ɵ)+MnO_2+2H_2O`
(c). `Hg^(2+)` and `Hg_2^(2+)` salts have `5d^(10)` configuration i.e., there is no d-d transition hence colourless.
(d). `Cu^(2+)` configuration is `3d^(9)` (one orbital is singly occupied) - Paramagnetic, Cu configuration is `3d^(10)` (all orbitals are doubly occupied)- Diamagnetic.