Question

Which one of the following is the weakest acid?

• A. $\mathrm{HF}$
• B. $\mathrm{HCl}$
• C. $\mathrm{HBr}$
• D. $\mathrm{HI}$

Answer 1

The correct option is A

$\mathrm{HF}$

Explanation for correct option

(A) $\mathbf{HF}$

Acid

• According to Arrhenius's theory, acids are species that dissociate in water to produce hydrogen ions, H+.
• However, two types of acid are found in nature: weak and strong.
• Weak acids are those that do not completely dissociate in an aqueous solution.
• Strong acids are those that dissociate entirely and readily when dissolved in an aqueous solution.

$\mathbf{HF}$

• Hydrogen fluoride $\left(\mathrm{HF}\right)$ is the only weak acid generated by hydrogen and a halogen.
• In water, $\mathrm{HF}$ do not entirely dissociate into their ions.
• As the size of halogen increases from fluorine to iodine, the bond strength between hydrogen and halogen decreases.
• The increasing order of hydrogen halide acidic strength is as follows:

$\mathrm{HF}<\mathrm{HCl}<\mathrm{HBr}<\mathrm{HI}$

Explanation for incorrect options

• Hydrogen fluoride is a weak acid.
• Hence, all other options (B), (C), and (D) are incorrect.

Therefore, the correct option is (A). $\mathrm{HF}$ is the weakest acid.