Question

Which one of the following is most acidic in water?
A. $NiC{{l}_{2}}$
B. $BeC{{l}_{2}}$
C. $FeC{{l}_{3}}$
D. $AlC{{l}_{3}}$

Answer 1

Hint: Salt of a strong acid and weak base form acidic solution when it is dissolved in water. $AlC{{l}_{3}}$ on hydrolysis gives a weak base $[Al{{(OH)}_{3}}]$ and a strong acid $(HCl)$ among all. That’s why $AlC{{l}_{3}}$is more acidic in water than any other compound.

Complete Step by Step Answer:
$AlC{{l}_{3}}$( ammonium trichloride ) is hygroscopic. This means it absorbs water from the air. $AlC{{l}_{3}}$ is made from the mixture of a weak base $[Al{{(OH)}_{3}}]$ and a very strong acid $(HCl)$ which is hydrochloric acid. Chloride ion does not react with water as it is a weak base whereas the cations of the salt is a weak acid which reacts with water.
$AlC{{l}_{3}}+{{H}_{2}}O\to Al{{(OH)}_{3}}+3HCl$

All other compounds also give us strong acid $(HCl)$and their corresponding salts $Ni{{(OH)}_{2}},\,Be{{(OH)}_{2}}\,\,and\,\,Fe{{(OH)}_{2}}$, but among them $[Al{{(OH)}_{3}}]$is the weakest base.

We know weak bases are the substances which do not completely dissolve in water and the acidic behaviour is due to the presence of hydrogen ions. In $(HCl)$, all the hydrogen ions and the chloride ions dissociate to produce ${{H}^{+}}$ ions in the absence of water.
Thus the solution of $AlC{{l}_{3}}$ is more acidic.
Thus, Option (D) is correct.

Note: We can find the compound whether it is acidic, basic or neutral with the help of its ions which dissociates in water. A solution can be called acidic if there are more hydrogen ions than the hydroxide ions and the solution is basic if there are more hydroxide ions than the hydrogen ions.