Complete and balance:
Cr2O72–(aq) + SO2(g) → Cr(OH)3(s) + SO42–(aq)
This reaction must be done in base. This is indicated by the presence of the chromium(III) hydroxide product (a weak base).
Oxidation half–reaction (S goes from +4 to +6):
SO2(g) + 2 H2O(l) → SO42–(aq) + 4 H+(aq) + 2 e–
S is mass balanced
O balance
H balance
charge balance