Let us see the structure of given compound melatonin
The two nitrogen atoms appear to be sp3 hybridized but the actual hybridization of both the nitrogens in the structure is sp2 as they have to gain planar geometry so that they can participate in the resonance making the compound more stable
in the given is buffer . so H2PO4- and HPO42- both are present with equal concentration . Na+ is spectator ion it is also present in the concentration higher than the given species above .
but PO4-3 is not present . so it is lowest concentration
