How many moles of oxygen are contained in one litre at S.T.P. of air if its volume content is 21% in air

Explanation

To solve the question of how many moles of oxygen are contained in one liter of air at standard temperature and pressure (S.T.P.) given that the volume content of oxygen in air is 21%, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Problem**: We need to find the number of moles of oxygen in 1 liter of air, knowing that oxygen makes up 21% of the volume of air.

2. **Calculate the Volume of Oxygen**:
- Since oxygen constitutes 21% of the air, we can calculate the volume of oxygen in 1 liter of air.
- Volume of oxygen = 21% of 1 liter = \( \frac{21}{100} \times 1 \, \text{L} = 0.21 \, \text{L} \).

3. **Use the Molar Volume at S.T.P.**:
- At S.T.P., 1 mole of any gas occupies a volume of 22.4 liters. This is known as the molar volume.

4. **Calculate the Number of Moles of Oxygen**:
- The number of moles (n) can be calculated using the formula:
\[ n = \frac{\text{Volume of gas}}{\text{Molar volume}} \]
- Here, the volume of oxygen is 0.21 L and the molar volume is 22.4 L.
- Thus, the number of moles of oxygen:
\[ n = \frac{0.21 \, \text{L}}{22.4 \, \text{L/mol}} \approx 0.0093 \, \text{moles} \]

5. **Final Answer**:
- Therefore, the number of moles of oxygen contained in one liter of air at S.T.P. is approximately **0.0093 moles**.