Question

Which of the following will give anhydrous $MgC{l_2}$ from hexahydrate?
(A) Hexahydrate is heated to ${175^ \circ }$ in vacuum
(B) The double salt $MgC{l_2}.N{H_4}Cl.6{H_2}O$ is evaporated to dryness and ignited.
(C) The crystals of hexahydrate are dehydrated in a current of dry $HCl$ gas.
(D) All of the above

Answer 1

Hint: Hexahydrate of $MgC{l_2}$ means magnesium chloride with six molecules of water, ${H_2}O$ . A compound or molecule is said to be anhydrous, when it contains no water molecules.

Complete solution:
Studies based on thermal analysis, X-ray diffraction, scanning electron microscope and chemical analysis of hexahydrate magnesium chloride showed that on thermal decomposition, it exhibits varying results. It showed that $MgC{l_2}.4{H_2}O$ will be formed when heating at ${69^ \circ }C$ , $MgC{l_2}.2{H_2}O$ will be formed when heated at ${129^ \circ }C$ , $MgC{l_2}.n{H_2}O$ $(1 \leqslant n \leqslant 2)$ will be formed when heated at ${167^ \circ }C$ and anhydrous form will be produced on further heating. So, the first option is correct.
We can obtain a dehydrated form of magnesium chloride hydrate, only to some extent by heating. It is not possible to obtain a fully dehydrated form. This is because, when we heat magnesium chloride in air, it undergoes hydrolytic decomposition. So, in order to avoid this, dehydration needs to be carried out in dry $HCl$ gas medium. But, again the problem with this method is that $HCl$ , being corrosive in nature is difficult to store and moreover a large amount of it gets utilized in the process. Therefore, an alternative approach has been discovered for the production of anhydrous magnesium chloride using ammonium magnesium chloride hexahydrate $(MgC{l_2}.N{H_4}Cl.6{H_2}O)$. Therefore, second and third statements are also correct.

So, the correct option is D.

Note: Magnesium ions play a vital role in many cellular activities, and hence magnesium chloride is often used as medicine, as a source of magnesium ions. Salts of magnesium chloride are readily soluble in water. The hydrated form of magnesium chloride can be easily extracted from sea water.