STP
Summary
STP means standard temperature and pressure: 0 °C (273 K) and 1 atm. At STP, one mole of any ideal gas occupies 22.4 L, so you can convert directly between gas volume and moles without using the full ideal gas law.
Detail
Meaning: STP is a fixed reference state for comparing gases: T = 273 K (0 °C) and P = 1 atm (760 mmHg or 760 torr). The ideal gas law ties volume to n, T, and P. When T and P are fixed, equal moles of any ideal gas take up equal volume. That volume is the molar volume, 22.4 L/mol.
Equation: V = nRT/P, where V is volume (m³, or L in most problems), n is moles (mol), R is the gas constant (8.314 J/(mol·K), or 0.0821 L·atm/(mol·K)), T is temperature (K), and P is pressure (Pa, or atm). Check: (1)(0.0821)(273)/1 ≈ 22.4 L.
On the MCAT: A passage gives a gas volume collected or consumed at STP, and you find moles with n = V / 22.4 L/mol, then use stoichiometry. You may also compare gas densities at STP: d = M / 22.4 g/L, so a larger molar mass means a denser gas. Avogadro's law questions use the same idea: equal volumes of gases at the same T and P hold equal numbers of molecules. If the conditions are not STP, switch to PV = nRT.
Common confusion: STP is not the same as the standard conditions used in thermodynamics (25 °C, 298 K, 1 M solutions, 1 atm for ΔG°, ΔH°, E°). The 22.4 L/mol value holds only at 0 °C and 1 atm. At 25 °C and 1 atm the molar volume is about 24.5 L. Also, always convert temperature to kelvin.
Sources
- Chemistry: The Central Science
- Chemistry (Zumdahl)
- Chemistry: The Molecular Nature of Matter and Change
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.