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Sulfur trioxide is like other nonmetal oxides. It is an acidic oxide. It reacts with water to form sulfuric acid, H2SO4(aq).

SO3(g) + H2O(l) ---->H2SO4(aq)

In the atmosphere, sulfur trioxide, SO3(g), dissolves in rainwater to form sulfuric acid, H2SO4. This rainwater solution contains an equilibrium mixture of sulfuric acid, H2SO4, and the ions, HSO41- , SO42-, H3O1+ and OH1-. This rainwater mixture can reach pH as low as 3 to 3.5 and is extremely acidic.

Sulfuric acid is a strong acid with two ionization steps. The ionization constants are dramatically different. The first ionization step is shown here with an ionization constant that is much bigger than "one" and favors formation of the products. --

--Ka1 = large
H2SO4(aq) + H2O(l) ----> HSO4-1(aq) + H3O1+(aq)

The second ionization step is shown here with an ionization constant Ka2 = 1.2 x 10-2 This is a smaller disssociation constant than the first. All polyprotic acids show this kind of decreasing acid strength for succeeding dissociation steps.

HSO4-1(aq) + H2O(l) ----> SO4-2(aq) + H3O1+(aq)

Where does the sulfur dioxide come from?

Fossil fuels typically contain some sulfur. When the sulfur bearing coal burns, the sulfur is oxidized along with the carbon. This means carbon dioxide and sulfur oxides are formed. Coal burning power plants, metal processing plants (smelters) release SO3 (g) in the waste gas products. Sometimes these acidic oxides are removed form the stack exhaust by "scubbers". The process essentially "scrubs" the exhaust gases by reacting the acid oxides before they reach the atmosphere. This is very important and minimizes release of these acidic oxides.