Question

The percentage composition of water in $MgC{{l}_{2}}.6{{H}_{2}}O$ is
A. 53.17
B. 67.84
C. 47.25
D. None of these

Answer 1

Hint: There is a formula to calculate the percentage composition by mass of a part of the compound which is present in the form of mixture and it is as follows.
The percentage composition by mass = $\dfrac{\text{Molar mass of the part of the compound}}{\text{Molar mass of the whole compound}}\text{ }\!\!\times\!\!\text{ 100}$

Complete Solution :
- In the question it is asked to calculate the percentage composition of water in $MgC{{l}_{2}}.6{{H}_{2}}O$ .
- The formula to calculate the percentage composition of water in $MgC{{l}_{2}}.6{{H}_{2}}O$ .
The percentage composition of water by mass = $\dfrac{\text{Molar mass of the water present in the compound}}{\text{Molar mass of the total compound}}\text{ }\!\!\times\!\!\text{ 100}$
- The molar mass of one water molecule is 18 g/mol.
- There are 6 water molecules in the given compound then the molar mass of the six water molecules = 108 g/mol.
- The molar of the whole compound $MgC{{l}_{2}}.6{{H}_{2}}O$ is 203 g/mol.

- Substitute the all the known values in the above formula to get the percentage of composition of water in $MgC{{l}_{2}}.6{{H}_{2}}O$ and it is as follows.
The percentage composition of water by mass
\[\begin{align}
  & =\dfrac{\text{Molar mass of the total water molecules present in the compound}}{\text{Molar mass of the total compound}}\text{ }\!\!\times\!\!\text{ 100} \\
 & \text{=}\dfrac{108}{203} \\
 & =53.17\text{ % }\!\!\!\!\text{ } \\
\end{align}\]
- The percentage composition of water in $MgC{{l}_{2}}.6{{H}_{2}}O$ is 53.17%.
So, the correct answer is “Option A”.

Note: The percentage composition of water in $MgC{{l}_{2}}.6{{H}_{2}}O$ is going to explain the amount of water percentage present in the given compound. We should know the percentage of water present in the given molecule while we are going to prepare a standard solution for the respective compound. If we don't consider the water present in the composition of the chemical the concentration of the solution prepared will be wrong.