##H_2SO_3##
Sulfurous and sulfuric acid are both acids formed from an oxide of sulfur. Generally, in chemical lingo anyway, the suffix -ous is used to describe the acid which has one less atom of oxygen than the -ic acid.
For example, ##color(green)”sulphuric”## acid is ##H_2SO_4″## and ##color(red)”sulphurous”## acid is ##H_2SO_3##.
