Avogadro's Law
Equal volumes contain equal numbers of molecules
Formula
V / n = constant
V₁ / n₁ = V₂ / n₂
- 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?
n₁ = 2.0 mol, V₁ = 5.0 L
n₂ = 2.0 + 3.0 = 5.0 mol, V₂ = ?
V₁/n₁ = V₂/n₂
V₂ = V₁ × (n₂/n₁) = 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