Twice the vapour density gives

Options

A. Actual vapour density
B. Relative vapour density
C. Molecular mass
D. Molar volume

Explanation

To solve the question "Twice the vapour density gives", we need to understand the concept of vapour density and its relation to molar mass.

### Step-by-Step Solution:

1. **Understanding Vapour Density**:
Vapour density (VD) is defined as the mass of a certain volume of gas compared to the mass of an equal volume of hydrogen at the same temperature and pressure. Mathematically, it can be expressed as:
\[ \text{Vapour Density} = \frac{\text{Density of gas}}{\text{Density of hydrogen}} \]

2. **Relative Vapour Density**:
When we talk about relative vapour density, it is often compared to hydrogen, which has a density of approximately 0.08988 g/L at standard conditions. For gases, the relative vapour density can be simplified to:
\[ \text{Relative Vapour Density} = \frac{m}{V} \div \frac{2}{V} = \frac{m}{2} \]
Here, \(m\) is the molar mass of the gas.

3. **Twice the Vapour Density**:
If we take twice the vapour density, we can express it as:
\[ 2 \times \text{Vapour Density} = 2 \times \frac{m}{2} = m \]
This shows that twice the vapour density is equal to the molar mass of the gas.

4. **Conclusion**:
Therefore, we can conclude that:
\[ \text{Twice the Vapour Density} = \text{Molar Mass} \]

### Final Answer:
Twice the vapour density gives the molar mass of the substance.