the "nitrogen rule" of mass spectrometry requires a compound containing an odd number of nitrogens to have an odd-mass molecular ion and a compound containing an even number of nitrogens to have an even-mass molecular ion.

what is the molecular formula of the chn-containing compound imidazole, m+ = 68?

C₃H₄N₂ is the molecular formula

Explanation:

The molecular ion peak is obtained at 68 units, which is the molecular mass of the compound. The even number of nitrogen atoms gives even mass of the molecule. In this problem, the mass is even so there is an even number of nitrogen atoms. It can be either two or four, from which it must be two nitrogen, as four nitrogen atoms will give higher mass. Now, we know that 28 out of 68 units belongs to nitrogen

The number of carbon atoms can also be determined in the same manner. Only one or two carbon atoms will leave much of the remaining units (28 or 16 units respectively) to hydrogen atoms. This leaves the option of three carbon atoms as any number higher than it will give molecular mass more than 68 units.

For hydrogen atoms, only 4 units are remaining this means four hydrogen atoms are present in the molecule. This is possible, if we consider that the compound is cyclic and contains two double bonds.

PS: If we consider two carbon atoms then there will be 16 units left which means sixteen hydrogen atoms. The valency of two carbon and two nitrogen atoms will only allow a maximum of eight hydrogen atoms in the molecule (giving only 60 units).