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: 2.5 - Dalton's law of partial pressures
Dalton's law is used to calculate the pressure of a gas.
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?
According to Dalton's Law: pCO + pCO2 = P = 1520 mm of Hg
= 0.49 atm.
Partial pressure of CO2, pCO2= P - (pCO)= 2.0 - 0.49 = 1147.6 mm of Hg