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Question:

Pb(NO3)2 on heating gives a brown gas which undergoes dimerisation on cooling. Identify the gas.

Solution:

Pb(NO3)2 on heating gives a brown gas nitrogen dioxide (NO2) which undergoes dimerisation on cooling to form dinitrogen tetraoxide (N2O4).
2Pb(NO3)2 673K → 4NO2 + 2PbO + O2