Partial Pressure Calculator

Partial pressure describes the pressure contribution made by one gas in a mixture. Dalton's law states that the total pressure of a mixture of nonreacting ideal gases equals the sum of the partial pressures of all components. Each gas behaves as though it alone occupies the container at the mixture temperature and volume. This concept is used in gas collection, respiratory physiology, atmospheric science, diving, combustion, and laboratory gas preparation. For an ideal mixture, a component's partial pressure equals its mole fraction multiplied by total pressure. Mole fraction is the component amount in moles divided by the total moles of every gas present. It is dimensionless and ranges from zero to one. A gas representing forty percent of the molecules therefore contributes forty percent of the total pressure. This calculator reports both values so the composition and pressure contribution can be checked together. All mole amounts must be counted on the same basis. Total mixture amount includes the selected component as well as every other gas. The component amount cannot exceed the total. Total pressure and partial pressure use the same unit; this page accepts atmospheres to keep the result unambiguous. The ratio itself is independent of whether amounts are stated in moles, millimoles, or proportional molecule counts, provided component and total use identical units. Dalton's law is especially useful when a gas is collected over water. The measured pressure then combines the desired gas pressure with water vapor pressure. Subtract the water vapor partial pressure at the collection temperature from the total pressure to obtain the dry gas pressure. In air, oxygen has a mole fraction close to 0.21, so its ideal partial pressure is about 0.21 atmosphere when total pressure is one atmosphere. At altitude, the oxygen fraction remains similar while lower total atmospheric pressure reduces oxygen partial pressure. Real gases depart from ideal behavior at high pressures, low temperatures, and conditions near condensation. Components may also react or interact strongly, making simple mole-fraction scaling less accurate. Humidity, changing temperature, dissolved gases, and instrument calibration can matter in practical measurements. For routine classroom calculations and moderate-pressure gas mixtures, Dalton's relationship is usually an effective model. Use measured fugacity or an appropriate equation of state when engineering accuracy is required outside the ideal range.