5 H2O + As2IIIO3 + 2 Br20 2 H3AsVO4 + 4 HBr-I

This is an oxidation-reduction (redox) reaction:

4 Br0 + 4 e- 4 Br-I (reduction)

2 AsIII - 4 e- 2 AsV (oxidation)

Change the oxidation states