Sodium chromate is prepared by fusing

Options

A. sodium dichromate with carbon
B. `(NH_(4))_(2)Cr_(2)O_(7)` with sodium hydrogen sulphate in introgen
C. chromite `(Fecr_(2)O_(4))` with sodium hydroxide in air.
D. chromite `(FeCr_(2)O_(4))` with sodium chloride in air.

Answer

C

Explanation

When chromite but, `FeCr_(2)O_(4)(FeO.Cr_(2)O_(3))`, is fused with NaOH in air, the Cr is oxidized to sodium chromate, a yellow solid:
`2FrCr_(2)O_(4) + 8NaOH + 3(1)/(2)O_(2)` `overset (1100C^(@)) rarr` `4Na_(2)CrO_(4) + Fe_(2)O_(3) + 4H_(2)O`
`Fe_(2)O_(3)` is insolublein water but sodium chromate is soluble. Thus the `Na_(2)CrO_(4)` is removed by dissolving it in water. It can be prepared by fusion of chromite ore with `Na_(2)CO_(3)`. `Na_(2)CrO_(4)` is quite solluble in water and is a strong oxidizing agen. The chromate ion, `CrO_(4)^(2-)`, can only exist in solution under alkaline conditions. `Na_(2)CrO_(4)` is stable in aqueous solution, since salt hydrolysis ensures that the solution is alkaline. In the laboratory, sodium chromate may be made by the oxidation of a `Cr^(3+)` salt with sodium peroxide in aqueous solution :
Oxidation
`2Cr^(3+) (aq) + 16OH^(-) (aq) rarr 2CrO_(4)^(2-) (aq) + 8H_(2)O(1) + 6e^(-)`
Reduction
`3O_(2)^(2-) (aq) + 6H_(2)O(1) + 6e^(-) rarr 12OH^(-) (aq)`
peroxide ion
The sodium peroxide provides sufficient `OH^(-)` ions in aqueous solution so it is not neccessary to add further alkali. Similarly, `Cr(OH)_(3)` on treatment with `H_(2)O_(2)` and NaOH produces `Na_(2)CrO_(4)`.