Partial Pressure Calculator
Calculate partial pressures using Dalton's Law (Pi = xi × P_total) or from the ideal gas law (P = nRT/V). Furthermore, enter any number of gas components by name and moles for a full multi-component mixture analysis. Each gas acts independently in an ideal mixture.
How to use the Partial Pressure Calculator
Method 1: Dalton's law — enter total pressure and moles of each component. Method 2: ideal gas law — calculate P for one component. Furthermore, both give partial pressure in atm.
List each gas: name moles (one per line). Furthermore, mole fractions are calculated automatically.
Enter moles, volume, and temperature for one component. Furthermore, P = nRT/V gives its partial pressure.
Partial pressures appear for all components. Moreover, the sum equals total pressure.
Air at 1 atm: O2 = 0.2095 atm. Furthermore, pO2 drives oxygen into the lungs and blood by diffusion.
Variants, options and when to use each
| Gas | Mole % | Partial P in air (atm) |
|---|---|---|
| N2 | 78.09 | 0.7809 |
| O2 | 20.95 | 0.2095 |
| Ar | 0.93 | 0.0093 |
| CO2 | 0.04 | 0.0004 |
The formula explained
xi = mole fraction of component i
P_total = total pressure
n, V, T = moles, volume (L), temperature (K)
Dalton's law states that each gas in a mixture exerts pressure as if it alone occupied the container. Furthermore, Pi = xi x P_total where xi = ni/n_total. The total pressure equals the sum of all partial pressures. Moreover, the ideal gas law gives the same result: Pi = niRT/V.
Worked example — dry air at 1 atm
| Gas | Mole % | P (atm) |
|---|---|---|
| N2 | 78.09% | 0.7809 |
| O2 | 20.95% | 0.2095 |
| Ar | 0.93% | 0.0093 |
| CO2 | 0.04% | 0.0004 |
What is partial pressure and Dalton's Law?
Partial pressure is the pressure a gas would exert if it alone occupied the same volume at the same temperature. Furthermore, Dalton's law states that total pressure = sum of partial pressures. Each ideal gas behaves independently — intermolecular forces between different species are neglected.Mole fraction xi = ni/n_total. Moreover, Pi = xi x P_total. The ideal gas law also gives Pi = niRT/V — both approaches give the same answer. Furthermore, partial pressure is proportional to mole fraction at constant T and P.
Partial pressures govern gas exchange in living systems. Additionally, O2 and CO2 diffuse according to partial pressure gradients across the alveolar membrane and cell walls. In diving, elevated gas partial pressures cause toxicity risks — oxygen toxicity at P_O2 > 1.6 atm; nitrogen narcosis at elevated P_N2.
Who uses this calculator?
Respiratory physiologists measure pO2 and pCO2 in arterial blood. Furthermore, anaesthesiologists calculate anaesthetic MAC from partial pressures. Chemical engineers design gas separations using partial pressure driving forces. Moreover, divers calculate safe partial pressures for gas mixtures at depth.
Historical context and related concepts
John Dalton published his law of partial pressures in 1801-1802. Furthermore, it supported his atomic theory — independent gas behaviour implied separate atomic identities. The physiological importance was recognised by Paul Bert (1878) who linked altitude sickness to reduced pO2. Moreover, clinical blood gas analysis (pO2, pCO2) became standard practice from the 1960s.
Why partial pressures govern respiratory physiology and diving safety
Oxygen therapy is prescribed by target pO2, not O2 percentage. Furthermore, at sea level 21% O2 gives pO2 = 0.21 atm; at 5000 m altitude the same 21% gives only 0.11 atm — insufficient for comfortable breathing. Moreover, divers face elevated N2 partial pressure causing narcosis and O2 toxicity — partial pressure calculations determine safe depth limits.Partial pressure in anaesthetic delivery systems
General anaesthesia uses volatile agents at specific partial pressures (MAC values). Furthermore, at altitude, the same vaporiser setting gives a lower partial pressure — anaesthesiologists must increase the vaporiser setting proportionally. Moreover, nitrous oxide (N2O) partial pressure determines its analgesic and anaesthetic effect — typically 50% N2O = 0.5 atm partial pressure for conscious sedation.
Frequently asked questions
Related tools
Ideal Gas Law Calculator
PV=nRT for single component gas. Furthermore, partial pressure follows from ideal gas law.
→Henry's Law Calculator
c = P/H uses partial pressure. Furthermore, Henry's law connects gas partial pressure to dissolved concentration.
→Significant Figures Calculator
Round partial pressures to 4 sig figs. Furthermore, atmospheric composition is known to 4 decimal places.
→Combined Gas Law Calculator
P1V1/T1 = P2V2/T2 for each component. Moreover, each gas component follows combined gas law independently.
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