6 FeIISO4 + K2Cr2VIO7 + 7 H2SO4 3 Fe2III(SO4)3 + Cr2III(SO4)3 + 7 H2O + K2SO4

This is an oxidation-reduction (redox) reaction:

6 FeII - 6 e- 6 FeIII (oxidation)

2 CrVI + 6 e- 2 CrIII (reduction)

Change the oxidation states