when plaster of paric comes in contact with water it sets into a hard mass. The composition of the hard mass is

Options

A. `CaSO_(4)*H_(2)O`
B. `CaSO_(4)*Ca(OH)_(2)`
C. `CaSO_(4)*2H_(2)O`
D. `CaSO_(4)*2Ca(OH)_(2)`

Explanation

To solve the question regarding the composition of the hard mass formed when plaster of Paris comes in contact with water, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Plaster of Paris**:
- Plaster of Paris is chemically known as calcium sulfate hemihydrate, with the formula \( \text{CaSO}_4 \cdot \frac{1}{2} \text{H}_2\text{O} \) or \( \text{CaSO}_4 \cdot \text{H}_2\text{O} \) (commonly referred to as \( \text{CaSO}_4 \cdot 1\text{H}_2\text{O} \)).

2. **Reaction with Water**:
- When plaster of Paris is mixed with water, it undergoes a chemical reaction where it rehydrates to form gypsum. The reaction can be represented as:
\[ \text{CaSO}_4 \cdot \text{H}_2\text{O} + \text{H}_2\text{O} \rightarrow \text{CaSO}_4 \cdot 2\text{H}_2\text{O} \]
- This indicates that one mole of plaster of Paris reacts with one mole of water to produce one mole of gypsum.

3. **Identifying Gypsum**:
- Gypsum is the dihydrate form of calcium sulfate, with the formula \( \text{CaSO}_4 \cdot 2\text{H}_2\text{O} \). It is a hard substance that sets when plaster of Paris is mixed with water.

4. **Conclusion**:
- Therefore, the composition of the hard mass formed when plaster of Paris comes into contact with water is \( \text{CaSO}_4 \cdot 2\text{H}_2\text{O} \), which corresponds to option C.

### Final Answer:
The composition of the hard mass is \( \text{CaSO}_4 \cdot 2\text{H}_2\text{O} \) (option C).

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