Which among the following is the least soluble?

The correct option is B

$\mathrm{LiF}$

Explanation for correct option:

(B) $\mathrm{LiF}$

• $\mathrm{Li}$ (Lithium) ions are small in size as compared to other ions of alkali metals.
• They have the highest energy of hydration but ions are bonded very strongly.
• Hydration energy is the total amount of energy released when one mole of ions is hydrated.
• The ions interact in a strong manner, this results in high lattice energy.
• Thus, they have high insolubility.

Explanation for incorrect options:

(A) ${\mathrm{BeF}}_2$

• It is soluble due to high hydration energy which can easily overcome the lattice energy.
• lattice energy is less than that of the energy of hydration.

(C) ${\mathrm{MgF}}_2$

• It is an inorganic compound.
• Therefore, it is insoluble in water.

(D) ${\mathrm{CaF}}_2$

• It has high lattice energy.
• Thus, it is insoluble in water.

Therefore the correct option is (B), $\mathrm{LiF}$.