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Water of Crystallization

How many wate...

Question

How many water molecules are present in one formula unit of the following compounds? a) Gypsum b) Plaster of Paris c) Epsom

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Solution

Water molecules are present in one formula unit of the following compounds are as follows:
a. Gypsum: 2 (two) [1 mark]
b. Plaster of Paris: 1/2 (half) [1 mark]
c. Epsom: 7 (seven) [1 mark]

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Q. Chemically, Plaster of Paris is calcium sulphate hemihydrate, i.e. containing half molecule of water of crystallisation. It is represented by the formula,

{CaSO}_{4} . \frac{1}{2} H_{2} O

. Half molecule of water of crystallisation means that one water molecule is shared by two formula units of

C a S O_{4}

. Hence we also represent its formula as

(C a S O_{4})_{2} . H_{2} O

. The name, Plaster of Paris, was given to this compound because for the first time, it was made from gypsum which was mainly found in Paris. Gypsum has the chemical formula,

C a S O_{4} .2 H_{2} O

. When plaster of Paris is mixed with water it sets into a hard mass.

(a) The number of water molecules present as water of crsytallisation in gypsum and Plaster of Paris respectively, is
(i)

2; 1 / 2

2; 5 / 2

1 / 2; 2

3 / 2; 1 / 2

(b) Write the chemical equation for the reaction between plaster of Paris and water.

Q. Q. Which of the following salts has two water molecules as water of crystallisation in its one formula units ? a) Plaster of paris b) Gypsum c) Blue Vitrol d) Caustic soda

Q. How many molecules of water of crystallisation (per formula unit) are present in:
(a) copper sulphate crystals?
(b) washing soda?
(c) gypsum?

Q. The number of water molecules in one molecule of gypsum, dead burnt plaster and plaster of paris, respectively

How many molecules of water of crystallisation (per formula unit) are present in (a) copper sulphate crystals (b) washing soda (c) gypsum ? [3 marks]

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