Discussion
If a mixture of NaCl, conc. H2SO4 and K2Cr2O7 is heated in a dry test tube, a red vapour (P) is formed. This vapour (P) dissolves in aqueous NaOH to form a yellow solution, which upon treatment with AgNO3 forms a red solid (Q). P and Q are, respectively
Answer: D
P is CrO2Cl2 and Q is Ag2CrO4.
This is the chromyl chloride test for chloride. Heating a chloride with dichromate and concentrated sulfuric acid gives chromyl chloride, a volatile deep red-brown liquid whose vapour distils out:
\[ \begin{aligned} &\mathrm{K_{2}Cr_{2}O_{7} + 4NaCl + 6H_{2}SO_{4}} \\ &\rightarrow \mathrm{2CrO_{2}Cl_{2} + 2KHSO_{4}} \\ &\quad + \mathrm{4NaHSO_{4} + 3H_{2}O} \end{aligned} \]
In NaOH it hydrolyses to yellow sodium chromate, Na2CrO4. Silver nitrate then precipitates brick-red silver chromate, Ag2CrO4.
The yellow solution holds chromate, not dichromate, so Ag2Cr2O7 is wrong. Na2[CrOCl5] is not volatile and is not formed in this test.
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