A container holds 0.5 mol N₂ and 0.5 mol O₂ at a total pressure of 100 kPa. What is the partial pressure of N₂?
A100 kPa — N₂ is the primary gas so it accounts for all the pressure
B50 kPa — N₂'s mole fraction is 0.5, so its partial pressure is 0.5 × 100 kPa
C25 kPa — the total pressure is split by molecular weight, not mole fraction
D75 kPa — O₂ is heavier and contributes less, so N₂ contributes more
Partial pressure equals mole fraction times total pressure: Pₙ₂ = xₙ₂ × P_total = 0.5 × 100 = 50 kPa. The mole fraction (ratio of moles of one species to total moles) is the correct weighting factor — molecular weight does not enter into it. This follows directly from the ideal gas law applied to each species independently: Pᵢ = nᵢRT/V.
Question 2 Multiple Choice
You replace 0.5 mol O₂ in a sealed container with 0.5 mol CO₂, keeping temperature and volume constant. How does the total pressure change?
AIt increases — CO₂ molecules are heavier, so they exert more pressure
BIt decreases — CO₂ and O₂ interact, reducing the partial pressures
CIt stays the same — total pressure depends only on total moles, T, and V, not on which gases are present
DIt increases slightly — adding a polyatomic gas always raises pressure
From P_total = n_total·RT/V, pressure depends on total moles, temperature, and volume — not on the identity of the gas molecules, as long as they behave ideally. Swapping 0.5 mol O₂ for 0.5 mol CO₂ leaves n_total unchanged, so P_total is unchanged. This is a direct consequence of molecular independence: each mole of ideal gas contributes identically to total pressure regardless of species.
Question 3 True / False
The partial pressure of a gas in a mixture equals the pressure it would exert if it alone occupied the same volume at the same temperature.
TTrue
FFalse
Answer: True
This is the definition of partial pressure: Pᵢ = nᵢRT/V. It is the pressure species i would exert in the container all by itself. Dalton's law then says total pressure is the sum of all such partial pressures — which follows because ideal gas molecules are completely indifferent to the presence of other species.
Question 4 True / False
Two gases that react chemically with each other can still be analyzed using Dalton's law of partial pressures, as long as both gases are ideal.
TTrue
FFalse
Answer: False
Dalton's law requires that the gases do not react. If they react, the mixture composition changes over time and you no longer have the original species at independent partial pressures — the chemical transformation alters the number of moles of each component. The assumption of molecular independence (the basis of Dalton's law) applies to non-reacting mixtures; ideal-gas behavior alone is not sufficient.
Question 5 Short Answer
Why does the presence of one ideal gas have no effect on the pressure exerted by another ideal gas in the same container?
Think about your answer, then reveal below.
Model answer: Ideal gas molecules are modeled as point particles with no intermolecular forces — they interact only through elastic collisions. Because molecules of gas A exert no attractive or repulsive forces on molecules of gas B, each species contributes to wall pressure exactly as if the other were absent. The pressure from each species depends only on its own mole count, temperature, and volume.
This molecular independence is the microscopic justification for Dalton's law. When intermolecular forces become significant (high pressure, polar gases), this independence breaks down and Dalton's law fails — real gas equations of state are needed instead.