Answer
a) $HF$ - hydrofluoric acid
b) $HCN$ - hydrocyanic acid
c) $HClO_{2}$ -chlorous acid.
Work Step by Step
Acids with a prefix of "hydro" are composed of the H+ cation and the negative anion which is in the name of the acid. Therefore, hydrofluoric acid is made up of H+ and F- anion, and hydrocyanic acid is made of H+ and the CN- anion.
Polyatomic ions with one fewer oxygen have the suffix “ous,” and thus to get the formula for chlorous acid we must look at the chlorite ion which is $ClO_{2}^-$ and thus join it with a $H^+$ to get $HClO_{2}$.
