How do gases share the total pressure?
You will be able to: Use mole fractions and Dalton’s law, including a water-vapor correction.
How do gases share the total pressure?
A container has 1 mol of helium and 3 mol of neon at one temperature. In an ideal mixture, each gas contributes to the pressure according to its particle count, not its share of the mass.
A useful starting point: What stays fixed when a gas variable changes? →
Words and symbols before equations
- Partial pressure Pᵢ
- Pressure gas i would exert alone in the same volume at the same temperature.
- Mole fraction Xᵢ
- Moles of component i divided by total gas moles; no units.
- Dalton’s law
- Total pressure is the sum of partial pressures for an ideal mixture.
- Water-vapor correction
- Subtract water’s partial pressure when finding the pressure of dry gas collected over water.
What this picture assumes
Ideal nonreacting He/Ne mixture, total n = 0.400 mol, V = 4.00 L, T = 300 K. Orange line = He; navy line = Ne. Symbols are representative counts, not literal molecule totals.
Read the picture in three steps
- Read the species and labels first. Identify what each symbol and line represents. Read the units and fixed conditions before comparing quantities.
- He: 0.6154 atm; Ne: 1.846 atm; sum = 2.462 atm. Total n,V,T stay fixed.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
For ideal nonreacting gases, each component obeys Pᵢ = nᵢRT/V. Adding component pressures gives Ptotal = ntotalRT/V.
Dividing the component equation by the total equation gives Pᵢ/Ptotal = nᵢ/ntotal = Xᵢ. Mole fraction is a count proportion, not a mass proportion.
If a collected sample includes water vapor, its measured total pressure includes that vapor. Use the supplied vapor pressure at the sample temperature: Pdry = Ptotal − Pwater. The dry gas and vapor share the same container volume.
A worked example, step by step
For 1.00 mol He plus 3.00 mol Ne at total pressure 2.40 atm, find both partial pressures.
- Total amount is 4.00 mol.
- XHe = 1.00/4.00 = 0.250; XNe = 0.750.
- PHe = 0.250 × 2.40 = 0.600 atm; PNe = 1.80 atm.
- Their sum is 2.40 atm. The heavier neon atoms do not add an extra mass-based pressure multiplier at equal temperature.
Use mole fraction, not mass fraction. Do not subtract water vapor unless water vapor is actually present.
A wet gas has total pressure 100 kPa and water-vapor pressure 3 kPa. What is Pdry?
Compare with an explanation
97 kPa. The measured 100 kPa includes both contributions.
Predict. Change one thing. Explain.
Keep total moles, volume and temperature fixed. Change helium’s mole fraction. Predict whether total pressure changes and check that partial pressures still add to it.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
He: 0.6154 atm; Ne: 1.846 atm; sum = 2.462 atm. Total n,V,T stay fixed.
Ideal nonreacting He/Ne mixture, total n = 0.400 mol, V = 4.00 L, T = 300 K. Orange line = He; navy line = Ne. Symbols are representative counts, not literal molecule totals.
Explain what you noticed: Answer the investigation prompt above. State one observation and explain it using particle interactions, concentration, gas behavior or energy transfer. Identify what the representation cannot tell you.
Apply the idea to a fresh problem Practice →Show what you understand.
Two original questions are a starting check, not proof of mastery. Explain your choice before revealing the answer.
Original written challenge
4 points · self-check · not an official AP questionGas is collected over water at total pressure 102 kPa. Water contributes 4 kPa. The dry portion contains 25% O₂ by moles and the rest N₂. Find the dry pressure and both dry-component partial pressures.
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Compare with the answer and four-point rubric
- 1 point: Pdry = 102 − 4 = 98 kPa.
- 1 point: PO₂ = 0.25 × 98 = 24.5 kPa.
- 1 point: PN₂ = 0.75 × 98 = 73.5 kPa.
- 1 point: Check 24.5 + 73.5 + 4 = 102 kPa and distinguish dry-mixture fractions from wet-mixture fractions.
Accept equivalent correct methods and explanations. This is a Refresh Kid teaching rubric, not an official AP scoring guideline.
Retrieve it before you reveal it.
RECALL 1Do mole fractions have units?
No; they are mole-count ratios.
RECALL 2What must partial pressures sum to?
The total pressure of the mixture.
RECALL 3Why subtract water vapor?
To remove its contribution from the measured wet-gas pressure.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
How do gases share the total pressure?
- Xᵢ = nᵢ/ntotal; Pᵢ = XᵢPtotal.
- Ptotal = ΣPᵢ; Pdry = Ptotal − Pwater.
Remember: Use mole fraction, not mass fraction. Do not subtract water vapor unless water vapor is actually present.
Conditions: Ideal nonreacting He/Ne mixture, total n = 0.400 mol, V = 4.00 L, T = 300 K. Orange line = He; navy line = Ne. Symbols are representative counts, not literal molecule totals.
Refresh Kid · AP Chemistry Unit 3 · Objectives 3.4.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 3.4, objective 3.4.A. CED effective Fall 2024 and June 2026 clarifications checked September 16, 2026. Unit 3: Properties of Substances and Mixtures, Topics 3.1–3.13. Focused lesson names, examples, models and assessments are original Refresh Kid teaching materials, not additional official topics or official AP questions. Official corrections.
The model states its assumptions beside the diagram. Colligative-property calculations and solution molality/mass-percent/volume-percent calculations are not required here. The optional speed-density model illustrates distributions; it does not require memorizing its mathematical derivation.
Teaching resources: The Organic Chemistry Tutor video titles/descriptions and topic coverage were checked for optional links; no claim is made to have watched every video. No creator scripts, examples, worksheets or artwork were copied. GitHub’s 3D website collection and its Three.js camera-control example informed the idea of controllable spatial inspection. Scientific diagrams, geometry and interactions here are original. The self-hosted Three.js runtime retains its MIT license. Camera rotation changes the view, not the chemistry.
Independent teacher review and observation of students remain pending. Implementation checks do not certify scientific accuracy, accessibility or learning effectiveness. This is a review edition.
Optional official resource: Released AP Chemistry questions and scoring guides. This archive contains questions across units; it is not an assignment of every question to this lesson.
The teaching sequence is informed by the IES learning guide; this exact implementation has not been evaluated with learners.
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