Question

The solubility of anhydrous $AlC{{l}_{3}}$ and hydrous $AlC{{l}_{3}}$ in diethyl ether are ${{S}_{1}}$ and ${{S}_{2}}$ respectively. Then-
(A) ${{S}_{1}}={{S}_{2}}$
(B) ${{S}_{1}}>{{S}_{2}}$
(C) ${{S}_{1}}<{{S}_{2}}$
(D) ${{S}_{1}}<{{S}_{2}}$ but not ${{S}_{1}}={{S}_{2}}$

Answer 1

Hint: Find out the molecular formula of anhydrous aluminium chloride and hydrous aluminium chloride. Think about which among the both will have more solubility in diethyl ether. Think about the Lewis acid concept and then think about the effect of hydration on solubility of a salt in ether.

Complete step by step answer:
We have studied about the properties of anhydrous compounds and also about the hydrous compounds and now let us focus on the aluminium chloride which are anhydrous and hydrous and relate their solubility in diethyl ether.
- Aluminium has the atomic number 13 and has three valence electrons. It needs 5 electrons to complete its octet. So, its valency is 5.
- Aluminium chloride, $AlC{{l}_{3}}$ is a salt of aluminium and chloride.
- Aluminium chloride is an electron-deficient species.
- Aluminium chloride is a Lewis acid. Aluminium chloride is hygroscopic in nature.
- Aluminium chloride in its hydrated form $AlC{{l}_{3}}.{{H}_{2}}O$ is a poor Lewis acid because it is very much stable as a lone pair of oxygen atoms in water can occupy the one vacant 2p orbital of aluminium atom.
- Water is immiscible in ether. So, aluminium chloride hydrate will repel ether solvent to some extent due to the presence of water molecules in it.
- Anhydrous aluminium chloride is a good Lewis acid. It is electron-deficient and so it can readily accept lone pairs of electrons from ether solvent and thus, get dissolved in ether easily.
- Therefore, solubility of anhydrous aluminium chloride is greater than solubility of hydrous aluminium chloride in diethyl ether solvent.
- Therefore, ${{S}_{1}}>{{S}_{2}}$. So the correct answer is “B”:

Note: Remember Lewis acid can easily accept electrons. If a molecule is present in its hydrate form then mostly it is very stable and cannot dissolve in non-polar solvents like ether. This is because the molecule attains a stable configuration and becomes poor Lewis acid. In case of anhydrous aluminium chloride, it still is electron-deficient and therefore readily accepts lone pairs of electrons from ether and gets dissolved.