3 Hg0 + 8 HNVO3 3 HgII(NVO3)2 + 2 NIIO + 4 H2O

This is an oxidation-reduction (redox) reaction:

2 NV + 6 e- 2 NII (reduction)

3 Hg0 - 6 e- 3 HgII (oxidation)

Change the oxidation states