Dalton proposed this law on the pressure exerted by a mixture of non-reacting gases in an enclosed vessel. The law of partial pressure states that the total pressure exerted by a mixture of two or more non-reacting gases in a definite volume is equal to the sum of the individual pressures, which each gas would exert if it occupies the same volume at a constant temperature. If p1, p2, p3 are the individual partial pressures of the known gases, then the total pressure 'P' of the mixture of gases at the same temperature and pressure is given by the relation:
P = p1 + p2 + p3 + …….
Partial pressure is the pressure exerted by the gas if present alone in the vessel at the same conditions of temperature.
For example,
A gas 'A' having a pressure of 300 mm Hg is contained in a vessel and another gas 'B' with a pressure of 400 mm Hg is contained in another vessel, are mixed in the third vessel at the same temperature. The total pressure in the third vessel is,
P = PA + PB
= 300 + 400 = 700 mm Hg
Fig: - Dalton's law of partial pressures
Dalton's law is used to calculate the pressure of a gas.
Problem
3. A 10 L flask at 298 K contains a gaseous mixture of CO and CO2 at a total pressure of 1520 mm of Hg. If 0.20 mole of CO is present, find the partial pressure of CO and that of CO2?
Solution
According to Dalton's Law: pCO + pCO22 = P = 1520 mm of Hg
= 0.49 atm.
Partial pressure of CO2, pCO2= P - (pCO)
= 2.0 - 0.49 = 1147.6 mm of Hg
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