Moist air is less dense than dry air at the same temperature and barometric pressure. Which is the best explanation for this observation?

$ \textbf{(A)}\hspace{.05in}\ce{H2O} \text{ is a polar molecule but } \ce{N2} \text{ and } \ce{O2} \text{ are not} \qquad$
$\textbf{(B)}\hspace{.05in} \ce{H2O} \text{has a higher boiling point than } \ce{N2} \text{or} \ce{O2}\qquad$
$\textbf{(C)}\hspace{.05in}\ce{H2O} \text{has a lower molar mass than} \ce{N2} \text{or} \ce{O2}\qquad$
$\textbf{(D)}\hspace{.05in}\ce{H2O} \text{has a higher heat capacity than} \ce{N2} \text{or} \ce{O2}\qquad$
1. **Understanding the Ideal Gas Law**: The ideal gas law is given by the equation:
   \[
   PV = nRT
   \]
   where \( P \) is the pressure, \( V \) is the volume, \( n \) is the number of moles, \( R \) is the ideal gas constant, and \( T \) is the temperature. For a given temperature and pressure, the number of moles \( n \) of gas in a fixed volume \( V \) is constant.

2. **Comparing Moist Air and Dry Air**: Both moist air and dry air are assumed to be at the same temperature and pressure. Therefore, according to the ideal gas law, they will contain the same number of moles of gas in a given volume.

3. **Molar Mass Consideration**: Dry air is primarily composed of nitrogen (\(\ce{N2}\)) and oxygen (\(\ce{O2}\)), with molar masses of approximately 28 g/mol and 32 g/mol, respectively. Moist air contains water vapor (\(\ce{H2O}\)), which has a molar mass of approximately 18 g/mol.

4. **Density Calculation**: Density (\(\rho\)) is defined as mass per unit volume. For a given number of moles \( n \) in a fixed volume \( V \), the mass \( m \) of the gas is given by:
   \[
   m = n \cdot M
   \]
   where \( M \) is the molar mass of the gas. Therefore, the density is:
   \[
   \rho = \frac{m}{V} = \frac{n \cdot M}{V}
   \]
   Since \( n \) and \( V \) are the same for both moist air and dry air, the density is directly proportional to the molar mass \( M \).

5. **Conclusion**: Because water vapor (\(\ce{H2O}\)) has a lower molar mass (18 g/mol) compared to nitrogen (\(\ce{N2}\)) and oxygen (\(\ce{O2}\)), the presence of water vapor in moist air reduces the average molar mass of the air mixture. Consequently, moist air has a lower density than dry air.

Thus, the best explanation for why moist air is less dense than dry air at the same temperature and barometric pressure is that \(\ce{H2O}\) has a lower molar mass than \(\ce{N2}\) or \(\ce{O2}\).

The final answer is \( \boxed{ \textbf{(C)}\hspace{.05in}\ce{H2O} \text{has a lower molar mass than} \ce{N2} \text{or} \ce{O2} } \)