Which of the following is the most acidic?
- AHClO₄
- BHClO₂
- CHClO
- DHClO₃
Show answer and explanation
Correct answer: (a) HClO₄
Explanation
The oxoacids of chlorine get stronger as the number of oxygen atoms increases:
HClO (+1) < HClO₂ (+3) < HClO₃ (+5) < HClO₄ (+7)
More oxygen atoms pull electron density away from the O–H bond, so the proton is released more easily. The ion left behind is also more stable: in the perchlorate ion (ClO₄⁻) the negative charge is spread over four oxygen atoms. So perchloric acid is the most acidic, and one of the strongest acids known.