# 5.6.2 Dalton’s Law of Partial Pressures

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,

pTotal = p1+p2+p3+......(at constant T, V)

where pTotal is the total pressure exerted by the mixture of gases and p1, p2 , p3 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.

pDry gas = pTotal – Aqueous tension

Partial pressure in terms of mole fraction

Suppose at the temperature T, three gases, enclosed in the volume V, exert partial pressure p1, p2 and p3 respectively, then,  x1 is called mole fraction of first gas.

Thus, p1 = x1 ptotal

Similarly for other two gases we can write

p2 = x2 ptotal  and  p3 = x3 ptotal

Thus a general equation can be written as

pi = xi ptotal

where pi and xi are partial pressure and mole fraction of ith 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.