The density of a gas at S.T.P. is 1.40 grams per litre. The molecular weight of the gas is

Options

A. 28
B. 30
C. 31.36
D. 35

Explanation

To find the molecular weight of a gas given its density at Standard Temperature and Pressure (S.T.P.), we can follow these steps:

### Step 1: Understand the given information
We are given the density of the gas at S.T.P. as 1.40 grams per litre. We know that at S.T.P., 1 mole of any gas occupies a volume of 22.4 litres.

### Step 2: Use the formula for density
Density (D) is defined as mass (m) per unit volume (V):
\[ D = \frac{m}{V} \]
From this, we can rearrange the formula to find the mass:
\[ m = D \times V \]

### Step 3: Substitute the known values
Substituting the values we have:
- Density (D) = 1.40 g/L
- Volume (V) = 22.4 L

So,
\[ m = 1.40 \, \text{g/L} \times 22.4 \, \text{L} \]

### Step 4: Calculate the mass
Now, we perform the multiplication:
\[ m = 1.40 \times 22.4 = 31.36 \, \text{grams} \]

### Step 5: Interpret the result
The mass we calculated (31.36 grams) corresponds to the molecular weight of the gas because at S.T.P., this mass represents one mole of the gas.

### Conclusion
Thus, the molecular weight of the gas is **31.36 grams per mole**.

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