What this calculator does
Each gas in a mixture exerts the pressure it would exert alone in the same container, and the total is their sum. That is Dalton law, and it holds because ideal gas molecules do not interact.
The consequence is that only the total number of moles matters for the total pressure, not how it is divided between species. Two moles as a three-gas mixture and two moles of a single gas give the identical 4.893 atm in a 10 litre container at room temperature. The identity of the gases is irrelevant.
The formula
Each gas partial pressure is its moles times RT over the volume, and the total is the sum, which is equivalent to using the total moles directly. R is 0.0820574 L·atm/(mol·K) so pressures come out in atmospheres. The ideal gas assumption fails at high pressure or low temperature, where real molecules occupy volume and attract each other.
| Term | Meaning |
|---|---|
| Partial pressure | The pressure one component would exert alone in the same container. |
| Dalton’s law | Total pressure is the sum of the partial pressures. |
| Mole fraction | Each partial pressure is the mole fraction times the total pressure. |
| Ideal gas | The assumption that molecules have no volume and do not attract each other. |
The inputs explained
| Field | What to enter |
|---|---|
| Moles of each gas, mol (comma-separated) | Moles of each gas, comma separated. |
| Temperature (K) | Temperature in kelvin. |
| Container volume (L) | Container volume in litres. |
When to use it
Analysing a gas mixture
Reaction mixtures and atmospheres are described by the partial pressure of each component.
Diving gas calculations
The partial pressure of oxygen and nitrogen at depth, not their percentages, determines physiological effect.
Equilibrium in the gas phase
Kp is expressed in partial pressures, so these are the quantities the expression needs.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does volume change the pressure?
The same gas mixture in different container volumes.
| Container volume | Total pressure | Total moles | Gas constant R used |
|---|---|---|---|
| 5 L | 9.786 atm | 2.000 mol | 0.082057 L·atm/(mol·K) |
| 10 L | 4.893 atm | 2.000 mol | 0.082057 L·atm/(mol·K) |
| 20 L | 2.447 atm | 2.000 mol | 0.082057 L·atm/(mol·K) |
Questions
What is Dalton’s law?
That the total pressure of a gas mixture is the sum of the pressures each component would exert alone in the same container. It follows from ideal gas behaviour, where molecules neither occupy volume nor attract each other, so each is unaffected by the presence of the others.
How do I find one gas partial pressure?
Multiply its mole fraction by the total pressure. A gas making up 25% of the moles exerts 25% of the pressure. Alternatively apply the ideal gas law to that gas alone using its own mole count, which gives the same answer.
Does the type of gas matter?
Not under the ideal gas assumption. Two moles of helium and two moles of carbon dioxide exert identical pressure in the same container at the same temperature. Real gases deviate, with heavier and more polar molecules departing furthest, but the deviations are small at ordinary conditions.
When does the ideal gas law fail?
At high pressure, where molecular volume becomes a meaningful fraction of the container, and at low temperature, where intermolecular attraction matters. Near condensation the errors become large. The van der Waals equation is the usual first correction.
For the composition it depends on, see the mole fraction calculator. For gas-phase equilibrium constants, see the Kp and Kc conversion calculator.