Vapour density of a volatile substance is `4(CH_(4)=1)`. Its molecular weight would be:

Options

A. `8`
B. `2`
C. `64`
D. `128`

Explanation

To find the molecular weight of a volatile substance given its vapor density, we can use the relationship between vapor density and molecular weight.

### Step-by-Step Solution:

1. **Understand the Relationship**:
The molecular weight (molar mass) of a gas can be calculated using the formula:
\[ \text{Molecular Weight} = 2 \times \text{Vapor Density} \]

2. **Identify Given Information**:
We are given the vapor density of the substance as 4 (relative to methane, which has a molecular weight of 16 g/mol).

3. **Calculate the Actual Vapor Density**:
Since the vapor density is relative to methane, we can find the actual vapor density of the substance:
\[ \text{Actual Vapor Density} = \text{Relative Vapor Density} \times \text{Vapor Density of Methane} \]
\[ \text{Actual Vapor Density} = 4 \times 8 = 32 \]
(Note: The vapor density of methane is 8 g/mol, as it is half of its molecular weight.)

4. **Calculate the Molecular Weight**:
Now, we can use the actual vapor density to find the molecular weight:
\[ \text{Molecular Weight} = 2 \times \text{Actual Vapor Density} \]
\[ \text{Molecular Weight} = 2 \times 32 = 64 \text{ g/mol} \]

5. **Conclusion**:
The molecular weight of the volatile substance is **64 g/mol**.