Answer
The formula of sulfurous acid is $H_{2}SO_{3}$.
Work Step by Step
Oxyacids with oxanions ending in -ite are named as : $ 'base name \hspace{1mm}of \hspace{1mm}oxyanion' + ous + acid$
Here, $sulfur$ is the basename and the oxyanion ends in $-ite$. so, the oxyanion is sulfite: $SO_{3}^{2-}$. To balance the charges, 2 hydrogen ions are present. So, the formula of sulfurous acid is $H_{2}SO_{3}$.