Answer
Perchloric acid: $HClO_{4}$
Cadmium: $Cd$
Cadmium (II) perchlorate: $Cd(ClO_{4})_{2}$
Work Step by Step
Perchlorate ion ($ClO_{4}^{-}$) has a -1 charge. To form an acid, hydrogen ions are needed. Hydrogen ion have a +1 charge ($H^{+}$). Since the charges are balanced, Perchloric acid is formed which has a formula $HClO_{4}$
In Cadmium (II) perchlorate, Cadmium has a +2 charge. Perchlorate ion ($ClO_{4}^{-}$) has a -1 charge. In order to balance charges, two perchlorate ions are required. Thus, the chemical formula of Cadmium (II) perchlorate is $Cd(ClO_{4})_{2}$
