Write the formula of perchloric acid

Perchloric acid is $\mathrm{HClO_4}$.

Draw its bonding pattern

In oxyacids of chlorine, the hydrogen is attached to oxygen, so the structure is of the type:

$\mathrm{HO{-}ClO_3}$

To minimize formal charges, chlorine forms:

• one single bond with the $\mathrm{-OH}$ oxygen,
• and three double bonds with the other three oxygens.

So the structure is:

$\mathrm{HO{-}Cl(=O)_3}$

Count the number of $\mathrm{Cl=O}$ bonds

From the above structure, there are three $\mathrm{Cl=O}$ double bonds.

Final answer

$\boxed{3}$

Comparison with stored correct answer

Stored correct answer = $3$

Our derived answer = $3$

Hence, the answer agrees with the stored correct answer.