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Question:

A metal 'M' reacts with nitrogen gas to give 'M3N'. 'M3N' on heating at high temperature gives back 'M' and on reaction with water produces gas 'B'. Gas 'B' reacts with aqueous solutions of CuSO4 to form a deep blue compound. 'M' and 'B' respectively are:

Ba and N2

Li and NH3

Na and NH3

Al and N2

Solution:

Li and NH3
The metal ‘M’ forms nitride M3N. Since N has valency 3, metal ‘M’ has valence 1. Hence, ‘M’ can be either Li or Na. So we can rule out Ba and Al.