Question

$HgC{l_2}$ is a covalent compound sparingly soluble in water, the solubility increases by the addition of chloride ions due to:
A) Common ion effect
B) Formation of a complex ${[HgC{l_4}]^{2 - }}$
C) Weakening of $Hg - Cl$ bonds
D) Strong ion-dipole forces.

Answer 1

Hint: We know that a covalent compound is the one which is formed by the sharing of valence electrons in order to complete the octet of each element present in the compound. Covalent compounds are insoluble in water but are soluble in organic solvents. $HgC{l_2}$ is a triatomic molecule where the $Hg$ atom is bonded with two $Cl$ atoms.


Complete solution:
When Chloride ions will be added to $HgC{l_2}$ ,the chemical equation will be written as:
$HgC{l_2} + 2C{l^ - } \to {[HgC{l_4}]^{2 - }}$
Here we see that formation of an ionic complex is taking place. Ionic compounds are more soluble in water than covalent compounds. Hence a water-soluble complex is obtained which increases the solubility of $HgC{l_2}$ when chloride ions will be added into it.
The increase in the solubility could not be because of the common ion effect as we know that when a common ion effect takes place the solubility of the compound always decreases. And also, for the common ion effect to occur the compound must be an ionic compound to form ion after dissociation and here $HgC{l_2}$ is a covalent compound.
Also, we know that covalent bonds are very strong and a large amount of energy is required to break a covalent bond so weakening of $Hg - Cl$ bond is also not the correct reason and also as $HgC{l_2}$ is not a ionic compound so the reason for the increase in solubility could not be strong ion-dipole forces.

Hence the option B is the correct answer.


Note: The solubility of $HgC{l_2}$ after the addition of chloride ions increases from $6\% $ at $20^\circ C$ to $36\% $ in $100^\circ C$. Also $HgC{l_2}$ is considered to be highly toxic in nature as any form of mercury when absorbed is poisonous as it has some corrosive properties which can cause internal damage to stomach, mouth and also cause throat ulcers.