Question

3.40 g of $CaS{{O}_{4}}$ (MW=136 g/mol) is formed when 4.30 g of hydrated calcium sulphate heated to a constant mass. How many moles of water of crystallization are combined with each mole of calcium sulphate?
A. 1
B. 2
C. 3
D. 4

Answer 1

Hint: Mole is represented by the symbol mol which is a unit of measurement for amount of substance in SI units where SI stands for International System of Units. A mole of substance or a mole of particles is defined as exactly equals to $6.02214076\times {{10}^{23}}$ particles and these particles may be atoms, molecules, ions or electrons.

Complete step by step answer:
- Water of crystallization can be defined as the water molecules that are present inside the crystals i.e. found in the crystalline framework of metal complexes or a salt which is not directly bonded to metal cation.
Hence to find out number of moles present in $CaS{{O}_{4}}$ we can consider $CaS{{O}_{4}} = \dfrac{3.40}{136} = 0.025mole$
Mass of ${{H}_{2}}O$ can be calculated by subtracting mass of anhydrous salt from the mass of hydrated salt, mass of hydrated salt is 4.30 g and mass of anhydrous salt i.e. $CaS{{O}_{4}}$ is 3.40 g given in the question by putting the values we got that
$Mass\ \text{of }{{H}_{2}}O = 4.30 - 3.40 = 0.90g$
- Number of moles of water molecules can be given as $\dfrac{0-0.90}{18} = 0.05$ mole
So we can say that 0.025 mole of $CaS{{O}_{4}}$ contains 0.05 mole of ${{H}_{2}}O$
$\therefore $1 mole of $CaS{{O}_{4}}$ contains $\dfrac{0.05}{0.025} = 2\ \text{mole of }{{\text{H}}_{\text{2}}}\text{O}$
Thus the formula of hydrated $CaS{{O}_{4}}$ is $CaS{{O}_{4}}.2{{H}_{2}}O$.
The correct answer is option “B” .

Note: Calcium sulphate also known by the name calcium sulfate represented by the chemical formula $CaS{{O}_{4}}$ is an inorganic compound. It has white solid which is hardly soluble in water and it causes permanent hardness in water.