Question

On heating gypsum at 373K it loses water molecules and forms a substance X. The percentage of water of crystallisation in X is

Answer 1

Answer:

12.41 %

Explanation:

The chemical formula of gypsum is CaSO₄.2H₂O

When gypsum is heated at 373 K, it loses its water of crystallisation and forms plaster of paris (CaSO₄.1/2H₂O) according to following chemical reaction

CaSO₄.2H₂O ↔ CaSO₄.1/2H₂O   +    1.5 H₂O

we need to calculate the percentage of water in plaster of paris

Molar mass of CaSO₄.1/2H₂O = 40 + 32 + 4 x 64 + 1/2 x 18

                                                 = 145 grams/mol

Molar mass of water = 18 grams/mol

Percentage of water (by mass) =

(Molar mass of water/Molar mass of CaSO₄.1/2H₂O) x 100

= (18/145) x 100

= 12.41 %