At `STP 5.6 "litre"` of a gas weigh `60 g`. The vapour density of gas is:

Options

A. `60`
B. `120`
C. `30`
D. `240`

Explanation

To find the vapor density of the gas given that at STP (Standard Temperature and Pressure) 5.6 liters of the gas weighs 60 grams, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the relationship between vapor density and molecular mass**:
The formula for vapor density (VD) is given by:
\[ \text{Vapor Density} = \frac{\text{Molecular Mass}}{2} \]
This implies that:
\[ \text{Molecular Mass} = 2 \times \text{Vapor Density} \]

2. **Calculate the number of moles in 5.6 liters of gas**:
At STP, 1 mole of gas occupies 22.4 liters. Thus, we can calculate the number of moles (n) in 5.6 liters using the formula:
\[ n = \frac{\text{Volume}}{\text{Molar Volume}} = \frac{5.6 \text{ liters}}{22.4 \text{ liters/mole}} = 0.25 \text{ moles} \]

3. **Determine the molecular mass of the gas**:
We know that the weight of 5.6 liters of the gas is 60 grams. To find the molecular mass, we can use the number of moles calculated:
\[ \text{Molecular Mass} = \frac{\text{Weight}}{\text{Number of Moles}} = \frac{60 \text{ grams}}{0.25 \text{ moles}} = 240 \text{ grams/mole} \]

4. **Calculate the vapor density using the molecular mass**:
Now that we have the molecular mass, we can find the vapor density:
\[ \text{Vapor Density} = \frac{\text{Molecular Mass}}{2} = \frac{240 \text{ grams/mole}}{2} = 120 \]

### Conclusion:
The vapor density of the gas is **120**.

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