The acidity or strength of an acid depends on its ability to donate hydrogen ions (H+) in water. The more easily an acid donates H+ ions, the stronger it is.

Sulfuric acid (H2SO4) is a strong acid because it completely dissociates in water, releasing two H+ ions per molecule:

H2SO4 → 2H+ + SO42-

This means that all of the H+ ions from sulfuric acid are available to react with other substances, making it a very strong acid.

Sulfurous acid (H2SO3), on the other hand, is a weak acid because it only partially dissociates in water, releasing one H+ ion per molecule:

H2SO3 → H+ + HSO3-

This means that only a small portion of the H+ ions from sulfurous acid are available to react with other substances, making it a weak acid.

The difference in acidity between sulfuric and sulfurous acid is due to the presence of the second oxygen atom in sulfuric acid. This additional oxygen atom helps to stabilize the negative charge on the sulfate ion (SO42-), making it more difficult for the H+ ions to be released. As a result, sulfuric acid is a much stronger acid than sulfurous acid.