Question

Among the following, which is insoluble in water?
A) Sodium fluoride
B) Barium fluoride
C) Calcium fluoride
D) Magnesium fluoride
E) All are correct.

Answer 1

Hint The answer is based on the fact that the solubility of the fluorides is based on the lattice energy that is the energy released when there is more attraction and more is the charge density. Find out which has the highest charge density and this will give you the answer.

Complete answer:
In the classes of inorganic chemistry, we have dealt with the various topics that deal with the calculation of oxidation numbers, valency of atoms, solubility and many other entities.
We shall now see how we can measure the solubility and insolubility of the fluorides.
- The solubility of fluorides is measured in the universal solvent that is water.
- Since water is polar solvent and thus only polar compounds will be soluble in water.
- We know that the alkaline earth metals are the divalent cations and they have a small radius. Because of this fact the charge density is so high that even the attraction between the atoms is also more.
- Due to this high attraction, the energy released will be more.
- In the above given options, compared to that of barium fluoride, magnesium fluoride has high lattice energy and also the calcium fluoride is sparingly soluble in water.
- Here, sodium is monovalent and therefore is not an alkaline earth metal and the calcium fluoride is sparingly soluble.
Thus, the correct answer would be magnesium fluoride because it has higher lattice energy compared to barium fluoride and barium fluoride is slightly soluble in water.

Therefore, the correct answer is option D) Magnesium fluoride.

Note: Note that sparingly soluble salts are those compounds which have the lower solubility and the name is given because it is used as a descriptive term for the parts of solvents needed to dissolve one part of the solute. Slightly soluble and sparingly soluble are the same.