Refrigeration and Air Conditioning
The total pressure exerted by the mixture of air and water vapour is equal to the sum of pressures which each constituent would exert, if it occupied the same space by itself. This statement is called

Newton's law of gases
Dalton's law of partial pressures
Avogadro's hypothesis
Kinetic theory of gases

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Refrigeration and Air Conditioning
The relative humidity is defined as

The ratio of actual mass of water vapour in a given volume of moist air to the mass of water vapour in the same volume of saturated air at the same temperature and pressure
The ratio of the actual mass of water vapour in a unit mass of dry air to the mass of water vapour in the same mass of dry air when it is saturated at the same temperature and pressure
The mass of water vapour present in 1 m3 of dry air
The mass of water vapour present in 1 kg of dry air

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