Which mixture is lighter than humid air ?

Options

A. `N_2+O_2+SO_2`
B. `N_2+O_2+CO_2`
C. `N_2+O_2+C_2H_6`
D. `N_2+O_2+He`

Answer

To determine which mixture is lighter than humid air, we need to analyze the components of humid air and compare them with other mixtures.

### Step-by-Step Solution:

1. **Understanding Humid Air**:
- Humid air is composed of nitrogen (N₂), oxygen (O₂), and water vapor (H₂O).
- The average molar mass of humid air is influenced by the presence of water vapor, which has a lower molar mass compared to nitrogen and oxygen.

2. **Calculating Molar Mass of Humid Air**:
- The molar mass of nitrogen (N₂) is approximately 28 g/mol.
- The molar mass of oxygen (O₂) is approximately 32 g/mol.
- The molar mass of water (H₂O) is approximately 18 g/mol.
- The average molar mass of humid air can be calculated by considering the proportions of these gases, but it is generally less than that of dry air due to the presence of H₂O.

3. **Identifying a Lighter Mixture**:
- To find a mixture that is lighter than humid air, we can consider the addition of lighter gases.
- Helium (He) has a molar mass of about 4 g/mol, which is significantly lower than that of water vapor.

4. **Constructing the Mixture**:
- A mixture of nitrogen (N₂), oxygen (O₂), and helium (He) would have a lower average molar mass than humid air because helium is lighter than water vapor.
- Therefore, the mixture N₂ + O₂ + He will be lighter than humid air.

5. **Conclusion**:
- The mixture that is lighter than humid air is N₂ + O₂ + He.

### Final Answer:
The mixture that is lighter than humid air is **N₂ + O₂ + He**.

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