Avogadro's law
Summary
Avogadro's law says that at constant temperature and pressure, the volume of a gas is directly proportional to its moles (V/n is constant). Equal volumes of different ideal gases at the same T and P hold equal numbers of particles, whatever the gas.
Detail
Meaning: At constant temperature and pressure, the volume of a gas is directly proportional to the number of moles present. More particles collide with the walls more often, so a flexible container expands until the pressure returns to its original value. Equivalently, equal volumes of different ideal gases at the same T and P contain the same number of particles. At STP (0 °C and 1 atm), one mole of an ideal gas occupies about 22.4 L.
Equation: V₁/n₁ = V₂/n₂, where V is gas volume (m³; liters are common) and n is amount of gas (mol). It is the n-dependence contained in PV = nRT.
On the MCAT: A passage describes a balloon, a piston or a gas syringe at fixed T and P, and you predict how the volume changes when gas is added, removed or produced by a reaction. You may also compare gas coefficients in a balanced equation, since at constant T and P the volume ratio of gases equals the mole ratio. For 2H₂ + O₂ → 2H₂O(g), three volumes of reactant gas become two. A graph of V against n is a straight line through the origin.
Common confusion: Students think a heavier or larger gas takes up more volume. Only the number of particles matters, not identity or mass, for an ideal gas. Also, the law needs constant T and P. In a rigid container, adding gas raises the pressure instead of the volume.
Sources
- Chemistry: The Central Science
- Chemistry: Atoms First
- General Chemistry: Principles and Modern Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.