Explain:
(a). Why is `K-2Cr_2O_7` generally preferred to `Na_2Cr_2O_7` in volumetric analysis?
(b). Which divalent metal ion has maximum paramagnetic character amont the first transition metals? Why?
(c). How the colour of `K_2Cr_2O_7` solution does depend on pH of the solution?

Explanation

(a). `K_2Cr_2O_7` is not hygroscopic ab nd can be used as primary standard.
(b). `Mn^(2+)`: it has maximum number of unpaired electrons `(n=6)`
(c). In the solution, the following equilibria exists:
`Cr_2O_7^(2-)+H_2Ounderset(larr)rarr2CrO_4^(2-)+2H^(ɵ)`
In acidic medium `(pHlt7)`, it exists as `Cr_2O_7^(2-)` ions and has orange colour while in basic mediu7m `(pHgt7)`, it exists as `CrO_4^(2-)` ions and has yellow colour.