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 assertion-reason question regarding the observed molecular mass of \( \text{CaCl}_2 \) being less than its ideal molecular mass, we can break it down step by step.

### Step 1: Understanding the Assertion
The assertion states that the observed molecular mass of \( \text{CaCl}_2 \) determined by any colligative property is less than the ideal molecular mass.

**Hint:** Recall that the ideal molecular mass is calculated based on the formula of the compound without considering any dissociation in solution.

### Step 2: Dissociation of \( \text{CaCl}_2 \)
When \( \text{CaCl}_2 \) is dissolved in water, it dissociates into its ions:
\[ \text{CaCl}_2 (s) \rightarrow \text{Ca}^{2+} (aq) + 2 \text{Cl}^{-} (aq) \]
This means that one formula unit of \( \text{CaCl}_2 \) produces three ions in solution: one \( \text{Ca}^{2+} \) ion and two \( \text{Cl}^{-} \) ions.

**Hint:** Remember that strong electrolytes like \( \text{CaCl}_2 \) dissociate completely in solution.

### Step 3: Van't Hoff Factor
The Van't Hoff factor (\( i \)) for \( \text{CaCl}_2 \) is calculated based on the number of particles produced in solution:
\[ i = 1 + 2 = 3 \]
This indicates that \( \text{CaCl}_2 \) contributes three particles to the solution.

**Hint:** The Van't Hoff factor is crucial in determining how colligative properties are affected by the number of particles in solution.

### Step 4: Colligative Properties and Molecular Mass
Colligative properties (like osmotic pressure, boiling point elevation, freezing point depression) depend on the number of solute particles in the solution. The observed molecular mass can be calculated using the formula for osmotic pressure:
\[ \Pi = i \cdot C \cdot R \cdot T \]
Where \( \Pi \) is the osmotic pressure, \( C \) is the concentration of the solution, \( R \) is the gas constant, and \( T \) is the temperature.

**Hint:** The observed molecular mass is inversely related to the number of particles in solution.

### Step 5: Calculation of Observed Molecular Mass
From the osmotic pressure equation, we can rearrange to find the observed molecular mass:
\[ \text{GMM} = \frac{i \cdot \text{given mass} \cdot R \cdot T}{\Pi \cdot \text{volume of solution}} \]
Since \( i \) is greater than 1 for \( \text{CaCl}_2 \), the observed molecular mass will be less than the ideal molecular mass (which assumes no dissociation).

**Hint:** The more particles that are present in solution, the lower the calculated molecular mass will be compared to the ideal value.

### Conclusion
Both the assertion and reason are correct. The assertion correctly states that the observed molecular mass of \( \text{CaCl}_2 \) is less than its ideal molecular mass, and the reason explains that this is due to the complete dissociation of \( \text{CaCl}_2 \) in water as a strong electrolyte.

### Final Answer
- **Assertion (A):** True
- **Reason (R):** True
- **Explanation:** The reason correctly explains the assertion.