Dalton's Law of Partial Pressures Calculator

At a glance:Dalton's law of partial pressures, proposed by the British chemist John Dalton in 1801, is a fundamental law describing the pressure behaviour of ideal gas mixtures. It states that in an ideal gas mixture with no chemical reaction, the total pressure equals the sum of the component partial pressures: P_total = P₁ + P₂ + P₃ + … (i.e. P_total = ΣPᵢ). A component's 'partial pressure' is the pressure that gas would exert if it alone occupied the entire container at the same temperature and volume. Dalton's law rests on the ideal-gas assumption: molecular volume is negligible and intermolecular forces are absent, so each gas acts as if the others were not there and independently presses on the container wall. This yields the practical relation Pᵢ = xᵢ × P_total, where the mole fraction xᵢ = moles of that gas / total moles of the mixture (between 0 and 1, with all mole fractions summing to 1). Using this tool's default: oxygen's mole fraction in air is about 0.21 and P_total ≈ 101.3 kPa, so oxygen partial pressure = 0.21 × 101.3 ≈ 21.3 kPa — roughly the oxygen partial pressure our lungs meet when breathing. Applications are wide: computing component partial pressures in gas mixtures (industrial gas production, atmospheric science, environmental monitoring); diving and high-altitude physiology, where elevated total pressure raises each gas's partial pressure (excess nitrogen causes nitrogen narcosis, excess oxygen causes oxygen toxicity) and altitude hypoxia means insufficient oxygen partial pressure — mixed-gas blending (e.g. heliox) is done by partial pressure; the 'collect-over-water' method, where the collected gas is saturated with water vapour and the dry-gas pressure = atmospheric − water vapour pressure at that temperature; equilibrium constants expressed in partial pressures (Kp); and gas calculations with the ideal-gas law PV = nRT. Caveats: it strictly holds for ideal gases and deviates at high pressure or low temperature where intermolecular forces matter; the gases must not react; xᵢ is a mole ratio, not a mass or volume ratio (though for ideal gases mole fraction equals volume fraction); partial and total pressure must share units (kPa, atm or mmHg); this calculator limits xᵢ to 0–1 and total pressure to non-negative. In short, this tool lets you quickly compute a component's partial pressure from its mole fraction and the total pressure — a practical aid for gas chemistry, diving physiology and gas-collection calculations.