What is the chemical combination formula of Plaster of Paris?
Try it first: pick an option, then read the explanation.
Understanding Chemical Formulas
Plaster of Paris is a building material derived from gypsum. Its chemical properties are defined by its base compound (Calcium Sulphate) and the amount of water chemically bonded to it (water of crystallization). Understanding the prefixes for the number of water molecules is key to identifying the correct formula.
Detailed Analysis of the Options
C. CaSO4 . 1/2H2O
This is the correct answer. This formula represents Calcium Sulphate Hemihydrate. The prefix "hemi-" means half, indicating that there is half a molecule of water for every molecule of calcium sulphate. This is the precise chemical composition of Plaster of Paris.
B. CaSO4 . 2H2O
This is incorrect. This formula represents Calcium Sulphate Dihydrate. The prefix "di-" means two. This is the chemical formula for gypsum, the raw material from which Plaster of Paris is made.
A. CaCO3 . 2H2O
This is incorrect. CaCO3 is Calcium Carbonate (limestone). The addition of water molecules in this format is not standard for limestone.
D. CaSO3 . 2H2O
This is incorrect. CaSO3 is Calcium Sulfite, a different chemical compound from Calcium Sulphate (CaSO4), which is the base for gypsum and plaster.
Summary: Gypsum vs. Plaster of Paris
| Common Name | Chemical Name | Formula |
|---|---|---|
| Gypsum | Calcium Sulphate Dihydrate | CaSO4 . 2H2O |
| Plaster of Paris | Calcium Sulphate Hemihydrate | CaSO4 . 1/2H2O |
Study Tips
- Hemi means Half: Remember that the prefix "hemi-" means half, as in "hemisphere." This directly links to the 1/2H2O in the formula for Plaster of Paris.
- Di means Two: The prefix "di-" means two. This links to the 2H2O in the formula for gypsum.
- Gypsum to Plaster: The process involves heating gypsum to remove water. So, the final product (plaster) must have less water than the starting material (gypsum). This helps you remember that plaster is 1/2H2O and gypsum is 2H2O.