millimeters of mercury (mm Hg) - sometimes called the torr, at sea level the pressure is 760 mm Hg or 760 torrs.

pascal (Pa) - defined in terms of force per unit area.

1 pascal (Pa) = 1 newton/meter²

bar - the equivalent of 100,000 Pa.

1 atm = 760 mm Hg = 101.325 kPa = 1.013 bar

compressibility - gases have no fixed volume.

Boyle's Law - the volume of a fixed amount of gas at a given temperature is inversely proportional to the pressure exerted by the gas.

P = C_B * 1/V or PV = C_B

Charles's Law - if a given quantity of gas is held at a constant pressure, its volume is directly proportional to the Kelvin temperature.

V = C_C * T or C_C = V/T

general gas law / combined gas law - the volume of a gas is inversely proportional to its pressure at constant temperature and directly proportional to the Kelvin temperature at constant pressure.

P₁ V₁ / T₁ = P₂ V₂ / T₂

Gay-Lussac's law of combining volumes - the ratio of the volumes of gases in a reaction was always a small whole number, as long as the volumes are measured at the same temperature and pressure.

Avogadro's hypothesis - equal volumes of gas under the same conditions of temperature and pressure have equal numbers of molecules.

Avogadro's Law - the volume of a gas, at a given temperature and pressure, is directly proportional to the quantity of gas.

V = CA * n

gas constant (R) - a constant used to interrelate the properties of any gas. R = 0.082057 L*atm / K*mol.

ideal gas law - describes the behavior of an ideal gas.

PV = nRT

standard temperature and pressure (STP) - Temperature of 273.15 K and a pressure of 1 atm.

standard molar volume - one mol of gas at STP occupies 22.414 L.

d = m/V = PM/RT