A sample of a chromium-containing alloy weighing 3.45 g was dissolved in acid, and all the chromium in the sample was oxidized to `CrO_(4)^(2-)` was then found that 3.0 g of `Na_2 SO_3` was required to reduce the `CrO_(2)^(-)` to `CrO_(4)^(2-)` in a basic solution, with `SO_(3)^(2-)`. getting oxidized to `SO_(4)^(2-)`. Then which of the following statement is incorrect?

Options

A. The percentage of chromium in alloy is 23.9%
B. Oxidation state of chromium changes from +6 to +4
C. 1.5 moles of `SO_(3)^(2-)` are consumed per mole of `CrO_(4)^(2-)`
D. One mole of base is involved to convert per mole of `CrO_(4)^(2-)` in this redox reaction.

Answer

B

Explanation

The reaction involved is: `2CrO_(4)^(2-) +3SO_(3)^(2-) +H_2 O to 2CrO_(2)^(-) + 3 SO_(4)^(2-) + 2OH^(-)`
In the reaction, the oxidation state of chromium changes from 16 to +3 and sulphur from +4 to +6. Also 1.5 mol of `SO_(3)^(2-)` are oxidized per mol of `CrO_(4)^(2-)` and one mole of base is used per mole of `CrO_(4)^(2-)` in the redox reaction.
mol of ` CrO_(4)^(2-) = 1.58 xx 10^(-2) mol CrO_(4)^(2-)`
Since there is one mole of Cr in each mole of `CrO_(4)^(2-)` then the above number of moles `CrO_(4)^(2-)` , is also equal to the number of moles of Cr that were present: 0.0158 mol `Cr xx 52.00 g mol^(-1) =0.825g Cr` in the original alloy.
% of Cr = (0.825 g Cr/3.450 g sample) 100%=23.9%Cr