Mixture Inputs
Blank components are excluded from the calculation.
Formula Used
Mole fraction: xᵢ = nᵢ ÷ ntotal
Partial pressure: Pᵢ = xᵢ × Pdry mixture
Ideal total pressure: Ptotal = nRT ÷ V
Dalton’s law states that each gas contributes pressure according to its mole fraction. When wet-gas correction is enabled, the calculator uses Pdry mixture = Ptotal − Pwater vapor.
How to Use This Calculator
- Choose a known pressure or the ideal-gas method.
- Enter each gas amount in moles. Leave unused components blank.
- Select units that match your measurements and preferred result display.
- Enable vapor correction only for a wet total pressure.
- Calculate, inspect the table, then download CSV or save a PDF.
Understanding Gas Mixture Partial Pressures
Why partial pressure matters
A gas mixture can have one total pressure. Each component still exerts its own pressure. That individual contribution is called partial pressure. The total pressure equals the sum of all component pressures when gases behave ideally.
Mole fraction controls the contribution. A component with half the mixture moles has a mole fraction of 0.5. At a dry mixture pressure of 100 kPa, that gas contributes 50 kPa. This relation makes composition changes easy to interpret.
Start with reliable composition data
Enter component amounts in moles whenever possible. Equal units are essential. You may use calculated moles from mass and molar mass. You may also use proportional amounts. For example, 78, 21, and 1 can represent dry air proportions. The calculator only uses relative amounts for mole fractions.
Leave inactive components empty. Do not enter negative values. Use enough significant figures for the equipment and purpose. Small trace gases can matter in sensitive detectors. They may be less important for basic classroom calculations. The result table separates composition from pressure, making reviews easier.
Known pressure versus ideal-gas pressure
Select known total pressure when a gauge, specification, or sensor already provides it. This method directly applies Dalton’s law. Select the ideal-gas method when you know total moles, temperature, and volume instead. The calculator applies P = nRT/V before distributing pressure across the components.
Absolute temperature is required for the ideal method. Celsius and Fahrenheit are converted to kelvin internally. Volume is also converted to cubic metres. Real gases can depart from ideal behavior at high pressures or near condensation. Use measured pressure or an appropriate equation of state in those cases.
Handling humid or wet gas samples
A wet gas sample contains water vapor. A total pressure reading then includes that vapor pressure. If your listed gases are dry components, subtract water vapor pressure first. The remaining dry-gas pressure is distributed by mole fraction. This method is common in gas collection over water and humidity-related measurements.
Do not subtract water vapor when water is already included as a mixture component. That would count the correction twice. Confirm that all pressures use absolute units where needed. Gauge pressure requires conversion to absolute pressure before ideal-gas calculations. Clear labels and consistent units prevent most errors.
Checking and exporting your result
Review the mole fractions first. They should add to one, allowing for rounding. Then add the partial pressures. They should equal the dry mixture pressure. The table also reports values in pascals for traceability. Switch display units to compare data with laboratory notes or equipment readings.
Download the CSV file for records, spreadsheets, or reports. Use the print control and select your browser’s PDF option for a compact result sheet. Keep source conditions with exported values. Temperature, volume, water vapor, and the pressure method can all affect interpretation. Good documentation supports repeatable physics calculations.
Example Data
| Gas | Amount (mol) | Mole Fraction | Partial Pressure at 101.325 kPa |
|---|---|---|---|
| Nitrogen | 0.78 | 0.78 | 79.0335 kPa |
| Oxygen | 0.21 | 0.21 | 21.2783 kPa |
| Argon | 0.01 | 0.01 | 1.0133 kPa |
Frequently Asked Questions
1. What is partial pressure?
Partial pressure is the pressure a single gas contributes within a mixture. Under ideal conditions, every component contribution adds to the total mixture pressure.
2. What law does this calculator use?
It uses Dalton’s law of partial pressures. Each gas pressure equals its mole fraction multiplied by the dry mixture pressure.
3. Can I enter mass instead of moles?
Convert each mass to moles first. Divide mass by molar mass, then enter the resulting amount. All gas amounts must use the same basis.
4. Why do mole fractions matter?
Mole fraction describes each component’s share of total gas moles. It determines the same component’s share of dry mixture pressure.
5. When should I use the ideal-gas option?
Use it when total moles, temperature, and volume are known, but mixture pressure is not. The calculator derives total pressure from those values.
6. Does the calculator accept Celsius?
Yes. Celsius, kelvin, and Fahrenheit are accepted for ideal-gas calculations. The calculator converts them internally to absolute temperature.
7. What is wet-gas correction?
Wet-gas correction removes water vapor pressure from a measured total pressure. The remaining dry pressure is allocated among the listed dry gases.
8. Should I subtract water vapor every time?
No. Subtract it only when total pressure includes humidity and water vapor is not entered as a component. Avoid double counting.
9. Do partial pressures always add to total pressure?
They add to dry mixture pressure when vapor correction is active. Otherwise, they add to the entered or calculated total pressure.
10. Are results suitable for high-pressure gases?
They are ideal-gas estimates. Dense gases, high pressures, and near-condensation conditions can require measured data, compressibility factors, or a real-gas equation.
11. How do I save my calculations?
Use Download CSV for spreadsheet-compatible data. Use Save Results as PDF, then choose your browser’s PDF destination in the print dialog.