Question

Which of the following is the correct chemical formula for Plaster of Paris?

• A. {4CaSO2 . 2H2O}
• B. {CaSO4 . 2H2O}
• C. {CaSO4 . 1/2 H2O}
• D. {3CaSO4 . 1/2 H2O}

Hint:

Remember the relationship: Gypsum ({CaSO4 . 2H2O}) has two water molecules. Heating it removes one and a half molecules, leaving Plaster of Paris ({CaSO4 . 1/2 H2O}) with "hemi" or half a water molecule.

Answer 1

The Correct Option is C

Step 1: Recall the source material of plaster. Plaster of Paris is made from gypsum. The chemical formula for gypsum is {CaSO4 . 2H2O} (calcium sulfate dihydrate).
Step 2: Understand the manufacturing process. To make Plaster of Paris, gypsum is heated to about 150°C, driving off most of the water. This process, called calcination, converts the calcium sulfate dihydrate into calcium sulfate hemihydrate. The prefix "hemi-" means half.
Step 3: Identify the correct formula. The chemical formula for calcium sulfate hemihydrate is {CaSO4 . 1/2 H2O}. When water is added back to plaster, it re-forms into gypsum and hardens.