Sunday, 27 November 2016


Dalton's Law of Partial Pressure and Its Applications

Dalton's Law of Partial Pressure
Statement: The total pressure exerted by a mixture of non-reacting gases is equal to the sum of their individual partial pressure. 
Mathematical Form: Let the gases are designated as 1,2 and 3 and their partial pressures are p1, p2 and p3 respectively. The total pressure Pt of the mixture of gases is given by:
Pt = p1 + p2 + p3
Applications of Dalton's Law of Partial Pressure
     Following are the applications of Dalton's Dalton's law of partial pressure.
(i) Collection of Gases Over Water: Some gases are collected over water in the laboratory. The gas during collection gathers water vapours and becomes moist. The pressure exerted by this moist gas is the sum of partial pressures of dry gas and that of water vapours. The partial pressure exerted by the water vapours is constant at a particular temperature and is called aqueous tension
Pmoist = pdry + pw vap
Pmoist = pdry + aqueous tension
pdry = Pmoist - aqueous tension
(ii) Process of Respiration: The process of respiration depends upon the difference in partial pressures. When animals inhale air then oxygen moves into the lungs as the partial pressure of oxygen in air is 159 torr, while the partial pressure of oxygen in the lungs is 116 torr. CO2 produced during respiration moves out in the opposite direction, as its partial pressure is more in the lungs that that in the air. 
(iii) Breathing at Higher Altitudes: At higher altitude, the pilot feels uncomfortable breathing because the partial pressure of oxygen in the un-pressurized cabin is low as compared to 159 torr, where one feels comfortable breathing.
(iv) Breathing in Depth of Sea: Deep sea divers take oxygen mixed with an inert gas say He and adjust the partial pressure of oxygen according to the requirement. Actually, in sea after every 100 feet depth, the diver experiences approximately 3 atm pressure, so normal air cannot be breathed in depth of sea. Moreover, the pressure of N2 increases in depth of sea and it diffuses in the blood. 

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