ANS:B - 29

To find the average molecular weight of air, we need to consider the molecular weights of its major components: nitrogen (N2) and oxygen (O2), which constitute the vast majority of air. The molecular weight of nitrogen (N2) is approximately 28 g/mol, and the molecular weight of oxygen (O2) is approximately 32 g/mol. The composition of dry air by volume is approximately 78% nitrogen and 21% oxygen. To find the average molecular weight of air, we can use the weighted average formula: Average molecular weight of air=(0.78×Molecular weight of nitrogen)+(0.21×Molecular weight of oxygen)Average molecular weight of air=(0.78×Molecular weight of nitrogen)+(0.21×Molecular weight of oxygen) =(0.78×28)+(0.21×32)=(0.78×28)+(0.21×32) =21.84+6.72=21.84+6.72 =28.56 g/mol=28.56g/mol Therefore, the average molecular weight of air is approximately 29 g/mol. So, the correct option is 29.