Dalton’s Law of Partial Pressures

Definition: Dalton’s Law of Partial Pressures states that the total pressure of a gas mixture is equal to the sum of all individual or partial pressures of its component gases.

P_total = P₁ + P₂ + P₃

Partial pressures are proportional to each gas’ mole fraction (how much of the gas is present in the mixture).

To find Mole Fraction:

To find the Partial Pressure for each gas in mixture:

P₁= (P_total) (X₁)    ….and so on for each gas in the mixture

To find Total Pressure: P_total = P₁ + P₂ + P₃

Problem: A sample of natural gas contains 8.24 moles of CH₄, 0.421 moles of C₂H₆, and 0.116 moles of C₃H₈. If the total pressure of the gases is 1.37 atm, what is the partial pressure of propane (C₃H₈)?

GIVEN:

Moles CH₄ = 8.24
Moles C₂H₆ = 0.421
Moles C₃H₈ = 0.116
P_Total = 1.37 atm

Part I: Find total number of moles

8.24 + 0.421 + 0.116 = 8.777 mols

Part II: Find the mole fraction for proprane (C₃H₈)

Part III: Find the partial pressure of proprane (C₃H₈)