Q74 / 180 · AIIMS 2003 · Chemistry
Assertion : Solution of $\mathrm{Na}_2 \mathrm{CrO}_4$ in water is coloured.
Reason: Oxidation state of $\mathrm{Cr}$ in $\mathrm{Na}_2 \mathrm{CrO}_4$ is +6 .
- AIf both assertion and reason are true and the reason is the correct explanation of the assertion
- Correct answerBIf both assertion and reason are true and the reason is not the correct explanation of the assertion
- CIf assertion is true but reason is false
- DIf both assertion and reason are false.
Solution
$\mathrm{Na}_2 \mathrm{CrO}_4$ solution in water is yellow in colour, when acidified changes to orange colour (due to the formation of $\mathrm{Cr}_2 \mathrm{O}_7^{2-}$ ions).
$$
\mathrm{Na}_2 \mathrm{CrO}_4+\mathrm{H}_2 \mathrm{SO}_4 \rightarrow \mathrm{Na}_2 \mathrm{Cr}_2 \mathrm{O}_7+\mathrm{Na}_2 \mathrm{SO}_4+\mathrm{H}_2 \mathrm{O}
$$
Oxidation state : $\mathrm{Na}_2 \mathrm{CrO}_4$
Let oxidation state of $\mathrm{Cr}$ be $x$.
$$
\Rightarrow 2(+1)+x+4(-2)=0 \Rightarrow x=+6 \text {. }
$$
Hence, the correct answer is option: 'B'.