In which of the following reaction, law of mass conservation is not valid

Options

A. `._(4)Be^(9)+ ._(2)He^(4)rarr ._(6)C^(12)+ ._(0)^(1)n`
B. `C+O_(2)rarr CO_(2)`
C. `CH_(4)+2O_(2)rarr CO_(2)+2H_(2)O`
D. `NaOH+HCl rarr NaCl+H_(2)O`

Answer

To determine in which of the given reactions the law of mass conservation is not valid, we need to analyze each reaction one by one. The law of mass conservation states that mass can neither be created nor destroyed in a chemical reaction. This means that the total mass of the reactants must equal the total mass of the products.

### Step-by-Step Solution:

1. **Identify the Reactions:**
- A: Beryllium + Helium → Carbon + Neutron
- B: C + O2 → CO2
- C: CH4 + 2O2 → CO2 + 2H2O
- D: NaOH + HCl → NaCl + H2O

2. **Analyze Reaction A: Beryllium + Helium → Carbon + Neutron**
- This is a nuclear reaction.
- In nuclear reactions, some mass is converted into energy, which means the total mass of the products is less than the total mass of the reactants.
- Therefore, the law of mass conservation is **not valid** for this reaction.

3. **Analyze Reaction B: C + O2 → CO2**
- Count the atoms:
- Reactants: 1 Carbon (C) + 2 Oxygen (O2) = 1 C + 2 O
- Products: 1 Carbon (CO2) + 2 Oxygen (CO2) = 1 C + 2 O
- The reaction is balanced, so it follows the law of mass conservation.

4. **Analyze Reaction C: CH4 + 2O2 → CO2 + 2H2O**
- Count the atoms:
- Reactants: 1 Carbon (C) + 4 Hydrogen (H4) + 4 Oxygen (O2) = 1 C + 4 H + 4 O
- Products: 1 Carbon (CO2) + 4 Hydrogen (2H2O) = 1 C + 4 H + 4 O
- The reaction is balanced, so it follows the law of mass conservation.

5. **Analyze Reaction D: NaOH + HCl → NaCl + H2O**
- Count the atoms:
- Reactants: 1 Sodium (Na) + 1 Oxygen (O) + 1 Hydrogen (H) + 1 Chlorine (Cl) = 1 Na + 1 O + 1 H + 1 Cl
- Products: 1 Sodium (Na) + 1 Chlorine (Cl) + 2 Hydrogen (H2O) = 1 Na + 1 Cl + 2 H + 1 O
- The reaction is balanced, so it follows the law of mass conservation.

### Conclusion:
- The only reaction where the law of mass conservation is not valid is **Reaction A: Beryllium + Helium → Carbon + Neutron**. This is due to the nature of nuclear reactions where mass is converted into energy.

### Final Answer:
**Option A: Beryllium + Helium → Carbon + Neutron**