Question

If air has 21% and 79% of Oxygen and Nitrogen by volume respectively. What is the average molecular weight?

• A. \( 29.3 \)
• B. \( 29 \)
• C. \( 28.84 \)
• D. \( 28 \)

Hint:

To compute average molecular weight of a gas mixture, multiply each component’s molecular weight by its mole or volume fraction, then sum the results.

Answer 1

The Correct Option is C

Step 1: Use the weighted average formula for molecular weight based on volume fractions. Given: \[ \text{Oxygen (O}_2): 21\%, \quad M = 32 \] \[ \text{Nitrogen (N}_2): 79\%, \quad M = 28 \] Step 2: Calculate average molecular weight: \[ M_{avg} = (0.21 \times 32) + (0.79 \times 28) = 6.72 + 22.12 = 28.84 \]