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JEE Main levelSingle correct optionDifficulty 2 of 3qa117

Exactly one of the following solids gives a colourless, inert gas (not supporting combustion) together with a green solid residue on heating. It is:

A $(NH X_{4}) X_{2} Cr X_{2} O X_{7}$
B $NH X_{4} Cl$
C $NH X_{4} NO X_{3}$
D $(NH X_{4}) X_{2} SO X_{4}$

Answer: A

(NH X_{4}) X_{2} Cr X_{2} O X_{7} N X_{2} + Cr X_{2} O X_{3} + 4 H X_{2} O

: nitrogen is oxidised from -3 to 0 and chromium reduced from +6 to +3, leaving green

Cr X_{2} O X_{3}

.

NH X_{4} NO X_{3}

gives

N X_{2} O

with no residue, and

NH X_{4} Cl

sublimes.

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