Question
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The percentage of water of crystallisation of a sample of blue vitriol is:
\[
  A.34.07\% \\
  B.35.07\% \\
  C.36.07\% \\
  D.37.07\% \\
 \]

Answer Verified Verified
Hint: We must know that the blue vitriol has crystalline copper sulfate having molecular formula. Cupric sulfate, CuSO4, commonly known as blue vitriol, is the most important salt of copper. It usually crystallizes as CuSO4 · 5H2O and has a bright blue colour.

Complete step by step answer:
Let’s start with understanding the concept of water of crystallization, the water of crystallization is defined as the water molecules that are present inside the crystal. During crystallization of a solution, these water molecules get incorporated inside the crystal leads to their presence inside the crystal. The water of crystallization can be removed by heating the sample but this might result in loss of crystalline property. For example, sodium chloride dehydrate is unstable at room temperature.
Now, coming back to the question, we are given blue vitriol which is also known as crystalline copper sulfate having molecular formula. Now, we have to calculate the molecular mass of which will be
Atomic mass of Copper, Sulphur, oxygen and water is \[63.5,{\text{ }}32,{\text{ }}16{\text{ }}and{\text{ }}18\] respectively.
Molecular mass of \[CuS{O_4}.5{H_2}O{\text{ }} = {\text{ }}63.5{\text{ }} + {\text{ }}32{\text{ }} + {\text{ }}\left( {4{\text{ }}X{\text{ }}16} \right){\text{ }} + {\text{ }}\left( {5{\text{ }}X{\text{ }}18} \right)\]
\[\;63.5{\text{ }} + {\text{ }}32{\text{ }} + {\text{ }}64{\text{ }} + {\text{ }}90\]
\[249.5\]
So, the molecular mass of blue vitriol is \[249.5g\] and out of which \[90g\] is water of crystallization.
So, the percentage of water of crystallization present in blue vitriol is the mass of water present divided by total mass and multiplied by 100.
\[\dfrac{{90}}{{249.5}} \times 100 = 36.07\% \]
So, the answer to this question is option C.\[36.07\% \].

Note:
We must know that a large number of crystals that are being formed at lab scale are having the water of crystallization in it. The important thing is that the water is present in a definite ratio in all these crystals. If in a crystal during formation the ratio is being changed then the crystal may not be formed or if formed will have some defects. Also, proteins crystallize with more water of crystallization as compared to inorganic salt.