π¨ Partial Pressure Calculator
Dalton's law for gas mixtures.
How to Use This Calculator
Enter the total pressure and each gas with its moles. The calculator returns mole fractions and partial pressures.
Dalton's Law
Pi = (ni / ntotal) x Ptotal
Mole fraction is the key: each gas's pressure share equals its molecule share. This works because ideal gas pressure depends only on particle count, not identity.
Worked Example
Air at 1 atm with 0.21 mol O2 and 0.79 mol N2: X(O2) = 0.21, P(O2) = 0.21 atm. X(N2) = 0.79, P(N2) = 0.79 atm. Sum = 1.00 atm.
Breathing and Partial Pressure
Your lungs care about partial pressures, not percentages. At sea level, oxygen's partial pressure of 0.21 atm drives oxygen into your blood. At 10,000 feet, total pressure drops to 0.69 atm and oxygen's partial pressure falls to 0.14 atm, which is why you feel breathless. Supplemental oxygen works by raising the inspired oxygen fraction, restoring the partial pressure your body expects.
Collecting Gas Over Water
A classic lab technique collects gas by bubbling it through water into an inverted container. But the collected gas is mixed with water vapor, so Dalton's law is needed to subtract the water's partial pressure (tabulated by temperature) and find the dry gas pressure. Forgetting this correction is one of the most common gas law lab errors.
Altitude Sickness
At 18,000 feet, oxygen's partial pressure is roughly half its sea level value, and most people develop altitude sickness without acclimatization. Your body compensates over days by producing more red blood cells. Climbers use supplemental oxygen above 26,000 feet where the partial pressure cannot sustain consciousness.
Decompression Sickness
Divers breathing compressed air absorb extra nitrogen proportional to its partial pressure at depth. Ascending too fast lets dissolved nitrogen form bubbles in blood and joints, causing the bends. Decompression stops let nitrogen diffuse out gradually. Helium mixtures reduce narcosis because helium is less soluble.
Frequently Asked Questions
What is Dalton's law?
The total pressure of a gas mixture equals the sum of each gas's partial pressure: Ptotal = P1 + P2 + ... Each gas behaves as if it alone occupied the container.
How do I find partial pressure from mole fraction?
Multiply the mole fraction by the total pressure: Pi = Xi x Ptotal. A gas that is 20 percent of the molecules contributes 20 percent of the pressure.
What is the partial pressure of oxygen in air?
Air is about 21 percent oxygen, so at 1 atm the oxygen partial pressure is 0.21 atm (160 mmHg). This is the number that matters for breathing, not the 21 percent itself.
Why do scuba divers care about partial pressures?
Nitrogen narcosis and oxygen toxicity depend on partial pressure, not percentage. At 30 meters (4 atm), air's oxygen partial pressure hits 0.84 atm, approaching risky levels for long dives.
Do gases react with each other in the mixture?
Dalton's law assumes they do not. It works well for noble gases, nitrogen, oxygen, and other non-reacting mixtures at ordinary pressures.