Chapter 11 · Gases
11.7Avogadro's Law: Volume and Moles
9 min · two checks
Predict
Equal volumes of ideal gases at the same temperature and pressure contain
The idea
- Apply V₁/n₁ = V₂/n₂.
- Explain why the gas’s identity drops out.
At the same temperature and pressure, one mole of any ideal gas occupies the same volume, because volume is set by how many particles are colliding, not by how heavy they are. Avogadro’s law: V₁/n₁ = V₂/n₂. Double the moles in a flexible container at constant P and T, and the volume doubles. This is why chemical equations can be read as volume ratios for gases at the same conditions: 2 volumes of hydrogen react with 1 volume of oxygen.
At 0 °C and 1 atm, the molar volume is about 22.4 L/mol. At 0 °C and 1 bar (100 kPa), it is about 22.7 L/mol. Use the value that matches the problem’s standard. Molar volume is a conversion factor between liters and moles only at those stated conditions. At other conditions, use the ideal gas law.
Keep these
- V is proportional to n at constant T and P.
- Equal volumes, equal moles, for ideal gases at the same T and P.
- 22.4 L/mol at 0 °C and 1 atm.