Question

What is the chemical formula of Plaster of Paris?

• A. \(CaSO_4 \cdot 2H_2O\)
• B. \(CaSO_4 \cdot \frac{1}{2}H_2O\)
• C. \(CaCO_3\)
• D. \(CaOCl_2\)

Hint:

Remember the relationship between gypsum and Plaster of Paris. Gypsum (\(CaSO_4 \cdot 2H_2O\)) loses water upon heating to form Plaster of Paris (\(CaSO_4 \cdot \frac{1}{2}H_2O\)).

Answer 1

The Correct Option is B

Step 1: Recall the composition of Plaster of Paris.
Plaster of Paris is a hemihydrate of calcium sulfate. This means that for every two molecules of calcium sulfate, there is one molecule of water of crystallization.
Step 2: Relate the composition to the chemical formula.
Calcium sulfate has the chemical formula \(CaSO_4\). The term "hemihydrate" indicates that the ratio of water molecules to calcium sulfate units is \( \frac{1}{2} \). Therefore, the chemical formula of Plaster of Paris is \(CaSO_4 \cdot \frac{1}{2}H_2O\). This can also be represented as \((CaSO_4)_2 \cdot H_2O\).
Step 3: Analyze the given options.
(1) \(CaSO_4 \cdot 2H_2O\) is the chemical formula of gypsum, which is calcium sulfate dihydrate. (2) \(CaSO_4 \cdot \frac{1}{2}H_2O\) is the chemical formula of Plaster of Paris, calcium sulfate hemihydrate. (3) \(CaCO_3\) is the chemical formula of calcium carbonate, commonly known as limestone, chalk, or marble. (4) \(CaOCl_2\) is the chemical formula of calcium hypochlorite, a component of bleaching powder.
Step 4: Identify the correct chemical formula for Plaster of Paris.
The chemical formula of Plaster of Paris is \(CaSO_4 \cdot \frac{1}{2}H_2O\).