total pressure

Meaning: Total pressure is the pressure that a mixture of gases exerts on the walls of its container. For an ideal gas, each component behaves as if it were alone in the container, so the total is the sum of the partial pressures (Dalton's law). Real gases follow this closely at low pressure and high temperature. A component's share of the total depends on its mole fraction.

Equation: Ptotal = P₁ + P₂ + P₃ + … = ntotal RT / V, with Pi = Xi Ptotal. Pi is the partial pressure of gas i (Pa), Xi = ni / ntotal is the mole fraction (unitless), ntotal is the total moles (mol), R = 8.314 J/(mol·K), T is temperature (K), and V is volume (m³).

On the MCAT: A common setup is a gas collected over water, where Ptotal = Pgas + PH₂O, so you subtract the water vapor pressure at that temperature to find the dry gas pressure. Other setups: two gases mixed in a fixed volume, or air at altitude. At altitude, total pressure falls while the O₂ fraction stays near 21%, so the partial pressure of O₂ falls.

Common confusion: Students think a heavier or larger gas contributes more pressure. At the same T and V, the contribution depends on moles, not on molar mass or identity. Adding an inert gas at constant V and T raises the total pressure by that gas's partial pressure and leaves the other partial pressures unchanged.