The law was formulated by John Dalton in 1801. It states that the total pressure exerted by the mixture of non-reactive gases is equal to the sum of the partial pressures of individual gases i.e., the pressures which these gases would exert if they were enclosed separately in the same volume and under the same conditions of temperature.

In a mixture of gases, the pressure exerted by the individual gas is called partial pressure. Mathematically,

*p*_{Total} =* p*_{1}+*p*_{2}+*p*_{3}+......(at constant* T*,* V*)

where *p*_{Total} is the total pressure exerted by the mixture of gases and *p*_{1}, *p*_{2} , *p*_{3} etc. are partial pressures of gases.

Gases are generally collected over water and therefore are moist. Pressure of dry gas can be calculated by subtracting vapour pressure of water from the total pressure of the moist gas which contains water vapours also.

Pressure exerted by saturated water vapour is called aqueous tension.

*p*_{Dry gas} =* p*_{Total} – Aqueous tension

*Partial pressure in terms of mole fraction*

Suppose at the temperature T, three gases, enclosed in the volume *V*, exert partial pressure *p*_{1}, *p*_{2} and *p*_{3} respectively, then,

*x*_{1} is called mole fraction of first gas.

Thus, *p*_{1} = *x*_{1} *p*_{total}

Similarly for other two gases we can write

* ^{p}*2

^{=}

*2*

^{x}*total*

^{p}^{and }

*3*

^{p}^{=}

*3*

^{x}*total*

^{p}Thus a general equation can be written as

* ^{p}*i

^{=}

*i*

^{x}*total*

^{p}where *p*_{i} *and x*_{i} are partial pressure and mole fraction of i^{th} gas respectively. If total pressure of a mixture of gases is known, the equation (5.29) can be used to find out pressure exerted by individual gases.