Law of mass action cannot be applied to

Options

A. Decomposition of gaseous HI
B. Decomposition of gaseous `PCI_5`
C. Transition of Rhombic Sulphur to Monoclicnic sulphur
D. Decomposition of Calcium Carbonate

Answer

To solve the question "Law of mass action cannot be applied to," we need to analyze each option provided and determine which one does not involve a chemical equilibrium where the law of mass action is applicable.

### Step-by-Step Solution:

1. **Understand the Law of Mass Action**:
The law of mass action states that the rate of a chemical reaction is directly proportional to the concentrations of the reactants raised to the power of their stoichiometric coefficients. This law is applicable only when a chemical equilibrium is established.

2. **Analyze Each Option**:
- **Option 1: Decomposition of Gases (e.g., HI)**:
- The decomposition of hydrogen iodide (2 HI ⇌ H2 + I2) is a chemical reaction that can reach equilibrium. Therefore, the law of mass action applies here.

- **Option 2: Decomposition of PCl5**:
- The decomposition of phosphorus pentachloride (PCl5 ⇌ PCl3 + Cl2) is also a chemical reaction that can establish equilibrium. Thus, the law of mass action applies here as well.

- **Option 3: Decomposition of Calcium Carbonate**:
- The decomposition of calcium carbonate (CaCO3 ⇌ CaO + CO2) is a chemical reaction that can reach equilibrium. Hence, the law of mass action applies here too.

- **Option 4: Transition of Rhombic Sulfur to Monoclinic Sulfur**:
- This transition involves a change in the physical state of sulfur without any chemical reaction occurring. It is a physical change, not a chemical reaction, and thus does not establish a chemical equilibrium. Therefore, the law of mass action does not apply here.

3. **Conclusion**:
Based on the analysis, the law of mass action cannot be applied to the transition of rhombic sulfur to monoclinic sulfur. Therefore, the correct answer is **Option 4**.

### Final Answer:
The law of mass action cannot be applied to the transition of rhombic sulfur to monoclinic sulfur.

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