Vapor pressure called partial pressure. For example, air at sea level, and saturated with water vapor at 20 °C, has partial pressures of about 2.3 kPa of water, 78 kPa of nitrogen, 21 kPa of oxygen and 0.9 kPa of argon, totaling 102.2 kPa, making the basis for standard atmospheric pressure. Vapor pressure is measured in the standard units of pressure. The International System of Units (SI) recognizes pressure as a derived unit with the dimension of force per area and designates the pascal (Pa) as its standard unit. One pascal is one newton per square meter (N·m or kg·m·s). Experimental measurement
Henry's law a species in the aqueous phase, and formula_8 is the partial pressure of that species in the gas phase under equilibrium conditions. The SI unit for formula_12 is mol/(m Pa); however, often the unit M/atm is used, since formula_17 is usually expressed in M (1 M = 1 mol/dm) and formula_8 in atm (1 atm = 101325 Pa). The Henry solubility can also be expressed as the dimensionless ratio between the aqueous-phase concentration formula_17 of a species and its gas-phase concentration formula_24: For an ideal gas, the conversion is: where formula_27 is the gas constant and formula_28 is the temperature.