ideal gases
Summary
An ideal gas is a model in which particles have negligible volume and no attractions or repulsions, so PV = nRT applies. Real gases behave most ideally at high temperature and low pressure, and small nonpolar gases such as He come closest.
Detail
Meaning: An ideal gas is a model in which particles have negligible volume, exert no attractive or repulsive forces on each other, and collide elastically. Because the particles act independently, P, V, n and T are linked by one simple law. Real gases come closest to this at high temperature and low pressure, where particles are far apart and fast.
Equation: PV = nRT, where P is pressure (Pa), V is volume (m³), n is amount of gas (mol), R is the gas constant (8.31 J/(mol·K)), and T is absolute temperature (K).
On the MCAT: Expect direction-of-change questions. At constant n and T, doubling P halves V (Boyle). At constant n and P, V rises in proportion to T in kelvin (Charles). Related topics are Dalton's partial pressures, Graham's law, and average kinetic energy depending on temperature alone. Another common question asks which real gas is closest to ideal: small, nonpolar gases such as He or H₂.
Common confusion: Students put Celsius into PV = nRT, but T must be in kelvin. Also, real-gas deviations come from attractions (PV/nRT below 1 at moderate pressure) and from particle volume (above 1 at very high pressure), so large or polar molecules deviate more.
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.