Molar Volume Formula
Volume occupied by one mole of gas
Formula
Vm = 22.4 L/mol (at STP)
V = n × Vm
- Vm = molar volume
- STP = 273.15 K, 1 atm
- V = total volume (L)
- n = number of moles
Derivation from Ideal Gas Law
PV = nRT
At STP: P = 1 atm, T = 273.15 K, R = 0.08206 L·atm/(mol·K)
V/n = RT/P = (0.08206 × 273.15) / 1 = 22.4 L/mol
Example
Problem: What volume does 3.5 moles of N₂ occupy at STP?
V = n × Vm = 3.5 mol × 22.4 L/mol = 78.4 L
Answer: 78.4 L
Important Notes
- Valid for ideal gases only; real gases deviate at high P or low T
- At non-STP conditions, use full ideal gas law PV = nRT
- All ideal gases have same molar volume at same T and P (Avogadro's hypothesis)
- Modern STP sometimes uses 273.15 K and 1 bar (100 kPa) → Vm ≈ 22.7 L/mol