Answer
Sulfur dioxide: $SO_{2}$
Water: $H_{2}O$
Sulfurous acid: $H_{2}SO_{3}$
Work Step by Step
Sulfur dioxide means two atoms of oxygen element are present along with one atom of sulfur, thus the chemical formula would be $SO_{2}$
Water has a hydrogen ion with a +1 charge and hydroxide ion has a -1 charge, the charges are balanced and water has a formula $H_{2}O$.
Sulfurous acid means it has a Sulfite ion and hydrogen ion. Sulfite ion has a -2 charge ($SO_{3}^{-2}$), to form an acid, hydrogen ions are required. Hydrogen ion has a +1 charge ($H^{+}$). In order to balance it, two hydrogen ions are required. Thus, the formula of sulfurous acid is $H_{2}SO_{3}$
