Chapter 10, Problem 185A

Gypsum is hydrated calcium sulfate. A 4.89-g sample of this hydrate was heated. After the water was removed, 3.87 g anhydrous calcium sulfate remained. Determine the formula for this hydrate and name the compound.

Interpretation Introduction

Interpretation:

Formula and name of the hydrated calcium sulfate needs to be determined for which 4.89 g hydrated sample gives 3.87 g anhydrous salt.

Concept introduction:

Hydrated salts are those which contain certain molecules of water as water of crystallization.

Answer to Problem 185A

Formula of the hydrate is CaSO₄.2H₂O. Name of the hydrate is calcium sulfate dihydrate.

Explanation of Solution

Mass of hydrated calcium sulfate is 4.89 g.

Mass of anhydrous calcium sulfate is 3.87 g.

Thus,

Mass of water $\text{=(4}\text{.89-3}\text{.87) g =1}\text{.02 g}\text{.}$

Molar mass of CaSO₄ is 136 g/mol.

Molar mass of H₂O is 18 g/mol.

Therefore, number of moles of calcium sulphate and water can be calculated as follows:

$n=\frac{m}{M}$

Putting the values,

Mole of CaSO₄$=\frac{3.87\text{ g}}{136\text{ g/mol}}=0.028\text{ mol}$

Mole of H₂O $=\frac{1.02\text{ g}}{18\text{ g/mol}}=0.056\text{ mol}$

Mole ratio of CaSO₄: H₂O $=0.028:0.056=1:2.$

So, the formula is CaSO₄.2H₂O.

Conclusion

Formula of the given hydrate is CaSO₄.2H₂O and it is called calcium sulfate dihydrate.