Avogadro's Law
Equal volumes contain equal numbers of molecules
Formula
V / n = constant
Vâ‚ / nâ‚ = Vâ‚‚ / nâ‚‚
- V = volume (L)
- n = number of moles
- Constant = at fixed T and P
- Volume directly proportional to moles
Key Principle
At constant temperature and pressure, doubling the number of gas molecules doubles the volume.
V ∠n (when T and P are constant)
Example
Problem: A balloon contains 2.0 mol of He at 5.0 L. If 3.0 mol more He is added at same T and P, what is the new volume?
nâ‚ = 2.0 mol, Vâ‚ = 5.0 L
nâ‚‚ = 2.0 + 3.0 = 5.0 mol, Vâ‚‚ = ?
Vâ‚/nâ‚ = Vâ‚‚/nâ‚‚
Vâ‚‚ = V₠× (nâ‚‚/nâ‚) = 5.0 × (5.0/2.0) = 12.5 L
Answer: 12.5 L
Applications
- Explains why balloons expand when inflated with more gas
- Basis for molar volume concept (22.4 L/mol at STP)
- Supports atomic theory: gases consist of discrete particles
- Combined with other gas laws forms ideal gas law: PV = nRT