Question

If the percentage of water of crystallisation in $MgS{{O}_{4}}.x{{H}_{2}}O$ is 13%, when what is the value of x?
A. 1
B. 4
C. 5
D. 7

Answer 1

Hint: As the percentage of water of crystallisation is given, try to use the formula for finding out the percentage of crystallisation and equate it with the value given in the question. The value of x will come out on solving the equation.

Complete answer:
Let us find out what water of crystallisation is. In chemistry, water of crystallization or water or hydration are water molecules that are present inside the molecular crystals. Water is often associated in the formation of crystals from aqueous solutions. In some contexts, crystallization is the total mass of water in a substance at a given temperature and is present in a given ratio, according to stoichiometry. Classically, "water of crystallization" refers to water that is found in the crystalline framework of a metal complex or a salt, which is not directly bonded to the metal cation.
Upon crystallization from water or moist solvents, many compounds associate water molecules in the framework of the crystals.. In the case of sodium chloride, the dihydrate is unstable at room temperature.
Proteins crystallize with large amounts of water in the crystal lattice. A water content of 50% is a general trend for proteins.
Now, let us calculate the percentage of water of crystallisation in the question. For that, we will find the mass percentage by water. So,
${{H}_{2}}O%=(\dfrac{wt\,of\,{{H}_{2}}O}{total\,wt\,of\,compound})\times 100%$, which is given to be 13%.
Now,
$(\dfrac{18x}{120+18x})\times 100=13$,
 18x is mass of crystallised water and denominator is total mass of compound

On solving the equation, we get the value of x as 1, which gives option A as our answer.

Note: There can be a definite amount of water of crystallisation present at a particular temperature. As the temperature rises, the tendency of the water evaporating out becomes more and thus, the compound reduces the water of crystallisation.