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Dalton's Law Formula

Dalton's law of partial pressure states that "the total pressure exerted by a mixture of gases is the sum of the partial pressure of the individual gases present." A partial pressure is the pressure that a gas in a mixture of gases would exert if it were present alone under the same conditions.

Dalton's Law Formula is mathematically expressed as
Ptotal = the total pressure of a gaseous mixture and
P1, P2, P3 and so on are the partial pressures of the individual gaseous components of the mixture.

Dalton's law describes the effect of gas composition on the pressures that the gas exerts on its surroundings.

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Dalton's Law Problems

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Solved problems based on Dalton's law are given below.

Solved Examples

Question 1: Three gases, Ar, N2 and H2 are mixed in a 500L container. Ar has a pressure of 255 torr, N2 has a pressure of 228torr and H2 has pressure of 752torr. What is the total pressure in the container?
According to Dalton's law the total pressure is the sum of the partial pressures of each gas. Add the three partial pressures together.

PAr = 255torr, PN2 = 228torr, PH2 = 752torr

Ptotal = PAr + PN2 + PH2 = 255torr + 228torr + 752torr

Ptotal = 1235torr


Question 2: Calculate the partial pressure of H2 at 20oC when the vapor pressure of water is 17.5torr. The total pressure of the gases is 750torr.
Ptotal = 750torr
PH2O = 17.5torr
PH2 = ?

Ptotal = PH2O + PH2
750torr = 17.5torr + PH2
PH2 = 732torr


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