Question

The strongest of the following acids, is

• A. $HBr$
• B. $HCl$
• C. $HF$
• D. $HI$

Answer 1

Answer: (D)

In the gaseous state the hydride are essentially covalent but in dilute aqueous solution the three hydrogen halides $HCl$, $HBr,HI$, are essentially fully ionised and are thus strong acids.
$HI$ is most strongly ionised and is thus the strongest acid.
$HF$ is the weakest acid among them, its ionisation constant being only $7.2\times 10^{-4}$.
This weakly acidic nature of $HF$ is largely due to very strong $H-F$ bond.
Decreasing order of the strength of acid is as follows $HI>HBr>HCl>HF$