how can nitrogen show +5 oxidation state as it does not have empty d-orbitals?
It can for sure. Being in the fifth group nitrogen has electronic configuration 1s2 2s2 2p3. Since it holds 5 electrons in the valance level it can give all of them to any atom with higher electronegativity. For instance, oxygen forms N2O5 and HNO3, which contain nitrogen showing +5 oxidation state.