A: Observed molecular mass of `CaCl_2` determined by any colligative property is less than ideal molecular mass.
R: `CaCl_2` ionised in water as it is strong electrolyte.

Answer

To solve the question, we need to analyze both the assertion (A) and the reason (R) provided.

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that the observed molecular mass of \( \text{CaCl}_2 \) determined by any colligative property is less than the ideal molecular mass.
- The ideal molecular mass of \( \text{CaCl}_2 \) can be calculated as follows:
- Calcium (Ca) has an atomic mass of approximately 40.08 g/mol.
- Chlorine (Cl) has an atomic mass of approximately 35.45 g/mol.
- Therefore, the molecular mass of \( \text{CaCl}_2 \) is:
\[ \text{Molar mass of } \text{CaCl}_2 = 40.08 + 2 \times 35.45 = 40.08 + 70.90 = 110.98 \text{ g/mol} \]

2. **Dissociation of \( \text{CaCl}_2 \)**:
- When \( \text{CaCl}_2 \) dissolves in water, it dissociates into ions:
\[ \text{CaCl}_2 \rightarrow \text{Ca}^{2+} + 2 \text{Cl}^- \]
- This means that one formula unit of \( \text{CaCl}_2 \) produces a total of 3 ions (1 \( \text{Ca}^{2+} \) ion and 2 \( \text{Cl}^- \) ions).

3. **Colligative Properties**:
- Colligative properties depend on the number of solute particles in a solution, not on their identity.
- Since \( \text{CaCl}_2 \) dissociates into 3 ions, the effective number of particles in solution is greater than 1.

4. **Effect on Observed Molecular Mass**:
- The observed molecular mass calculated from colligative properties (like osmotic pressure, boiling point elevation, etc.) will be lower than the ideal molecular mass because the presence of more particles (3 instead of 1) means that the same amount of solute contributes to a larger number of particles.
- Thus, the observed molecular mass is given by:
\[ \text{Observed molecular mass} = \frac{\text{Ideal molecular mass}}{\text{Number of particles}} = \frac{110.98}{3} \approx 36.99 \text{ g/mol} \]
- This confirms that the observed molecular mass is indeed less than the ideal molecular mass.

5. **Understanding the Reason (R)**:
- The reason states that \( \text{CaCl}_2 \) ionizes in water as it is a strong electrolyte.
- This statement is correct because strong electrolytes completely dissociate into their ions in solution.

6. **Conclusion**:
- Both the assertion (A) and reason (R) are correct, and the reason correctly explains the assertion.

### Final Answer:
- **Assertion (A)** is true: The observed molecular mass of \( \text{CaCl}_2 \) determined by any colligative property is less than the ideal molecular mass.
- **Reason (R)** is true: \( \text{CaCl}_2 \) ionizes in water as it is a strong electrolyte.