Case 1
- Mole fraction x (0 to 1)
- 0.21
- Total pressure (kPa)
- 101.325kPa
21.278kPa
Open with these valuesPartial Pressure Calculator
21.278kPa
Result: 21.278 kPaDalton's law: a gas contributes its mole fraction of the total pressure. Oxygen at a mole fraction of 0.21 in air at 101.325 kPa has a partial pressure of 21.278 kPa. The mole fraction has no unit, so the answer comes back in whatever pressure unit you entered.
The numbers at a glance
Held fixed: Mole fraction x (0 to 1) 0.2100.
| Total pressure (kPa) (kPa) | Result (kPa) |
|---|---|
| 0.000 | 0.000 |
| 50.000 | 10.500 |
| 100.000 | 21.000 |
| 101.325Your value | 21.278 |
| 150.000 | 31.500 |
| 200.000 | 42.000 |
Worked examples
Case 2
- Mole fraction x (0 to 1)
- 0.78
- Total pressure (kPa)
- 101.325kPa
79.034kPa
Open with these valuesCase 3
- Mole fraction x (0 to 1)
- 0.5
- Total pressure (kPa)
- 200kPa
100.000kPa
Open with these valuesHow it's calculated
P_i = x × P_total
- StepEnter the mole fraction of the gas — its share of all the molecules, from 0 to 1.
- StepEnter the total pressure of the mixture; sea-level air is 101.325 kPa.
- ResultRead the partial pressure in the same unit as the total.
Reference table
| Mole fraction, total pressure | The gas | Partial pressure in kPa |
|---|---|---|
| 0, 101.325 | a gas that is not in the mixture | 0.000 |
| 0.21, 101.325 | oxygen in dry air at sea level | 21.278 |
| 0.78, 101.325 | nitrogen in dry air at sea level | 79.034 |
| 0.5, 200 | half of a mixture at 200 kPa | 100.000 |
| 1, 101.325 | a pure gas — the whole pressure | 101.325 |
Questions
How do I calculate a partial pressure?
Multiply the mole fraction of the gas by the total pressure of the mixture: P_i = x × P_total. Oxygen at x = 0.21 in air at 101.325 kPa gives 21.278 kPa.
What is Dalton's law of partial pressures?
The total pressure of a mixture of non-reacting gases is the sum of the pressures each gas would exert alone. That is the same as saying every gas takes its mole fraction of the total, so all the partial pressures add back up to it.
What is the mole fraction here?
The moles of that one gas divided by the total moles of all gases, a number between 0 and 1. Dry air is about 0.78 nitrogen and 0.21 oxygen.
Which pressure unit does the answer use?
The one you entered, because the mole fraction is dimensionless. Standard atmospheric pressure is 101.325 kPa, which is 1 atm or 760 mmHg.
Sources and last check
Information, not professional advice.