TFS · Thermodynamics · Problem 9PDFSolution in PDF ↓
TFS · Thermodynamics · Problem 9
Problem & Solution
Video Synthesis
- Problem: A room at atmospheric pressure contains 72 degree air and 80% relative humidity.
- Key step: We make the assumption that the relationship between nitrogen and oxygen in that room is 79% nitrogen and 21% oxygen.
- Watch out: They use the same variables I'm using but they don't really explain, you know, they don't explain a whole lot in the reference handbook.
- Result: The mole fraction of water vapor is equal to the partial pressure of water vapor divided by the total pressure in the space.
- ✅ Answer: B
Office Hours
1
Student questions asked in live office hours about this problem
OH 119 · April 28, 2026
Q: When working with gas mixtures like moist air, what is the relationship between mole fractions and mass fractions, and which should you calculate first?
A: For ideal gases, mole fractions equal volume fractions — if a gas is 79% nitrogen by molecule count, it also occupies 79% of the volume. Mass fractions differ because heavier molecules (O₂ ≈ 32 g/mol vs. N₂ ≈ 28 g/mol) contribute disproportionately more mass, so oxygen's mass fraction exceeds its mole fraction. Always start with mole fractions since they are often given directly as gas composition percentages, then convert to mass fractions using molar masses only when the problem requires it.