ideal gas law
Summary
The ideal gas law, PV = nRT, links the pressure, volume, amount and absolute temperature of a gas whose particles have negligible volume and no attractions. Most testable: T must be in kelvin. Real gases deviate most at high pressure and low temperature.
Detail
Meaning: The law describes an idealized gas whose particles take up negligible volume, exert no attractive or repulsive forces on each other, and collide elastically. Real gases behave this way closely at low pressure and high temperature. Holding some variables fixed gives the named gas laws: Boyle (P ∝ 1/V at constant n and T), Charles (V ∝ T at constant n and P), Gay-Lussac (P ∝ T at constant n and V) and Avogadro (V ∝ n at constant T and P). For a fixed amount of gas changing conditions, P₁V₁/T₁ = P₂V₂/T₂.
Equation: PV = nRT. P is pressure (Pa; atm is common), V is volume (m³; L is common), n is moles (mol), T is absolute temperature (K), and R is the gas constant, 8.314 J/(mol·K), or about 0.0821 L·atm/(mol·K).
On the MCAT: A passage changes the conditions of a gas, and you predict the direction, such as pressure rising when a rigid container is heated. You may also calculate moles, density (d = PM/RT) or molar mass, or use about 22.4 L/mol at 0 °C and 1 atm. Questions on deviations ask why a real gas departs from ideality.
Common confusion: Students plug in °C. Temperature must be in kelvin, so going from 25 °C to 50 °C does not double the volume. Also match the units of R to the units of P and V.
Sources
- Chemistry: The Central Science
- Chemistry (Zumdahl)
- General Chemistry (Ebbing and Gammon)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.