Question

What is the formula of plaster of Paris?

• A. $ \text{CaSO}_4 $
• B. $ \text{CaSO}_4 \cdot \frac{1}{2} \text{H}_2\text{O} $
• C. $ \text{CaSO}_4 \cdot \text{H}_2\text{O} $
• D. $ \text{CaSO}_3 $

Hint:

Plaster of Paris is a specific form of calcium sulfate hemihydrate, with the formula $ \text{CaSO}_4 \cdot \frac{1}{2} \text{H}_2\text{O} $. It is commonly used in construction and medical applications due to its rapid setting properties when mixed with water.

Answer 1

The Correct Option is B

Step 1: Understand Plaster of Paris.
Plaster of Paris is a form of calcium sulfate hemihydrate, which is obtained by heating gypsum ($ \text{CaSO}_4 \cdot 2\text{H}_2\text{O} $). Its chemical formula is: \[ \text{CaSO}_4 \cdot \frac{1}{2} \text{H}_2\text{O}. \] Step 2: Analyze Each Option.
Option (1): $ \text{CaSO}_4 $ — Incorrect, as this is the formula for anhydrous calcium sulfate, not plaster of Paris.
Option (2): $ \text{CaSO}_4 \cdot \frac{1}{2} \text{H}_2\text{O} $ — Correct, as this matches the formula for plaster of Paris.
Option (3): $ \text{CaSO}_4 \cdot \text{H}_2\text{O} $ — Incorrect, as this is the formula for gypsum, not plaster of Paris.
Option (4): $ \text{CaSO}_3 $ — Incorrect, as this is the formula for calcium sulfite, not plaster of Paris. Step 3: Final Answer. \[ \boxed{(2) \quad \text{CaSO}_4 \cdot \frac{1}{2} \text{H}_2\text{O}} \]