Partial Pressure Calculator (Dalton's Law)
Calculate the partial pressure of each gas in a mixture from the total pressure and each component's mole fraction (or moles), using Dalton's Law of Partial Pressures: P_i = x_i × P_total.
Tips
- In moles mode, just enter each component's moles — mole fractions are computed automatically and always add up to 1.
- In mole-fraction mode, if the fractions don't add up to exactly 1, they're automatically normalized, so small input errors won't break the calculation.
- You can add or remove between 2 and 5 components. Try it with the main constituents of air (nitrogen, oxygen, argon, carbon dioxide).
- For a single gas, use the companion Ideal Gas Law calculator (PV=nRT) to explore how pressure, volume, temperature, and moles relate.
Frequently Asked Questions
Side Note — When partial pressure becomes a matter of life and death
Dalton's Law of Partial Pressures was proposed in 1801 by the English chemist John Dalton, better known for atomic theory. Dalton had a strong interest in meteorology, and while studying how water vapor and other gases behave in the atmosphere, he found that even when several gases are mixed together, each behaves independently, and the total pressure is simply the sum of each gas's partial pressure.
The air we breathe is itself a mixture: roughly 78% nitrogen, 21% oxygen, 0.93% argon, and 0.04% carbon dioxide. At sea level, where total pressure is about 1 atmosphere, oxygen's partial pressure is roughly 0.21 atm — and it is this partial pressure, not the percentage alone, that governs how efficiently oxygen is absorbed into the bloodstream.
The concept of partial pressure is also central to medicine and sports science. Altitude sickness sets in as total atmospheric pressure drops with elevation, lowering oxygen's partial pressure, while scuba diving increases total pressure with depth, raising nitrogen's partial pressure and driving more of it into the bloodstream — the root cause of decompression sickness ("the bends"). Understanding partial pressure correctly is not just a chemistry exercise; it is directly tied to physical safety.