Sulfur trioxide, SO3 (g), reacts with water to form sulfuric acid, H2SO4 (g), a major component of volcanic smog. What is the balanced chemical equation for this reaction?

SO3 is sulphur trioxide. This is the second oxidation step in the oxidation of sulphur in the contact process for sulphuric acid production

Step 1: S + O2 → SO2

Step 2 2SO2 + O2 → 2SO3

The SO3 gas combines with water to form H2SO4

SO3 + H2O → H2SO4