Atmospheric composition — What is the average molecular weight (apparent molar mass) of dry air typically used in engineering calculations?

Introduction:The average molecular weight of dry air is required in numerous calculations, including gas densities, flow metering, and psychrometrics.

Given Data / Assumptions:

• Dry air composition approximated as ~78% N₂ (28 g/mol), ~21% O₂ (32 g/mol), with small fractions of Ar (40 g/mol) and CO₂ (44 g/mol).
• Humidity neglected (dry basis).

Concept / Approach:The apparent molecular weight is a mole-fraction-weighted average of component molar masses. Using standard atmospheric proportions yields a value close to 28.96 g/mol, commonly rounded to 29 g/mol for engineering work.

Step-by-Step Solution:Approximate calculation: M_air ≈ 0.7828 + 0.2132 + 0.01*40 ≈ 21.84 + 6.72 + 0.40 ≈ 28.96 g/mol.Round to the standard engineering value: 29 g/mol.Use 29 g/mol in density and molar flow computations unless high precision with real composition is needed.

Verification / Alternative check:Textbook tables and property databases list dry air ≈ 28.96 g/mol (often rounded to 29 g/mol).

Why Other Options Are Wrong:

• 21 and 23 are too low; 79 confuses with nitrogen volume percent; 32 corresponds to oxygen’s molar mass.

Common Pitfalls:Using 28 g/mol (nitrogen) instead of the mixture average; neglecting argon and CO₂ contributions when high accuracy is required.

Final Answer:29