K2Cr2VIO7 + 3 H2S-II + 4 H2SO4 K2SO4 + Cr2III(SO4)3 + 3 S0 + 7 H2O

This is an oxidation-reduction (redox) reaction:

3 S-II - 6 e- 3 S0 (oxidation)

2 CrVI + 6 e- 2 CrIII (reduction)

Change the oxidation states