Pᵢ = χᵢ × P_total (Dalton's Law)
Dalton's law states that the total pressure of a gas mixture equals the sum of partial pressures: P_total = P₁ + P₂ + … Each gas's partial pressure Pᵢ = χᵢ × P_total, where χᵢ is its mole fraction. Air at 1 atm: P(N₂) = 0.78 atm, P(O₂) = 0.21 atm, P(Ar) = 0.009 atm, P(CO₂) = 0.0004 atm. Partial pressure determines: gas solubility (Henry's law: C = kH × P), equilibrium constants (Kp), respiratory gas exchange (alveolar O₂ ≈ 100 mmHg, CO₂ ≈ 40 mmHg), and anesthesia dosing. When collecting gases over water, subtract water vapor pressure: P_gas = P_total − P_H₂O. Dalton's law holds exactly for ideal gases and approximately for real gases at moderate pressures.