The number of moles of nitrogen present in one litre of air containing 20% nitrogen by volume, under standard condition, is:

Options

A. 4.91 x `10^(-3)`
B. 8.92 x `10^(-3)`
C. 4.91 x `10^(-2)`
D. 8.92 x `10^(-2)`

Explanation

To find the number of moles of nitrogen present in one liter of air containing 20% nitrogen by volume under standard conditions, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Volume of Nitrogen**:
- Given that air contains 20% nitrogen by volume, we can calculate the volume of nitrogen in 1 liter of air.
- Volume of nitrogen = 20% of 1 liter = \( \frac{20}{100} \times 1 \) liter = 0.20 liters.

2. **Use the Ideal Gas Law**:
- Under standard conditions (STP), one mole of an ideal gas occupies 22.4 liters.
- We will use this information to find the number of moles of nitrogen in 0.20 liters.

3. **Calculate the Number of Moles**:
- The formula to calculate the number of moles (n) is given by:
\[ n = \frac{\text{Volume of gas}}{\text{Molar volume at STP}} \]
- Substituting the values:
\[ n = \frac{0.20 \text{ liters}}{22.4 \text{ liters/mole}} \]

4. **Perform the Calculation**:
- Calculate the number of moles:
\[ n = \frac{0.20}{22.4} \approx 0.00893 \text{ moles} \]

5. **Express in Scientific Notation**:
- The number of moles can be expressed in scientific notation:
\[ n \approx 8.93 \times 10^{-3} \text{ moles} \]

### Final Answer:
The number of moles of nitrogen present in one liter of air containing 20% nitrogen by volume under standard conditions is approximately \( 8.93 \times 10^{-3} \) moles.

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