Dalton's Law of Partial Pressures

In an ideal gas mixture, partial pressure p_i is mole fraction x_i times total pressure: p_i=x_i·p_total. Mole fraction is x_i=n_i/n_total. If oxygen makes up 0.21 of a mixture at total pressure 100 kPa, its partial pressure is 21 kPa. Add all partial pressures; their sum must equal total pressure. The law describes gases in the same volume and at the same temperature and is an approximation for real mixtures. Check whether the mixture is dry or humid and whether all components share the same available volume. At high pressures, an ideal partial-pressure model may no longer describe real interactions accurately.

Total pressure is the sum of contributions from all gas components. In an ideal mixture, a gas’s partial pressure equals the pressure it would exert alone at the same temperature in the available volume. It is therefore a calculated share, not a separate container pressure. For dry air near sea level, p_O₂ is about 0.21·101 kPa, or roughly 21 kPa. This component pressure can be crucial for solubility, reaction equilibria and gas transport. Check whether the mixture is dry or humid and whether all components share the same available volume. At high pressures, an ideal partial-pressure model may no longer describe real interactions accurately.

For ideal gas mixtures, p_i/p_total=x_i=n_i/n_total. This lets you calculate a component pressure from a known composition or infer composition from partial pressures. Example: p_A=30 kPa and p_B=70 kPa give p_total=100 kPa and x_A=0.30. Check that fractions sum to one and pressures to the total. For non-ideal mixtures, fugacities may better describe the effective chemical behavior. Check whether the mixture is dry or humid and whether all components share the same available volume. At high pressures, an ideal partial-pressure model may no longer describe real interactions accurately.

Yes. Gas collected over water usually also contains water vapour. The measured total pressure is then the sum of the dry gas partial pressure and water-vapour pressure. Thus p_gas=p_total−p_H₂O at known temperature, since water vapour pressure depends on temperature. Only then should the ideal gas law or a mole fraction be applied. Assigning the whole pressure to the dry gas overestimates its amount. Check whether the measurement is reported as dry or wet. Check whether the mixture is dry or humid and whether all components share the same available volume. At high pressures, an ideal partial-pressure model may no longer describe real interactions accurately.

Dalton’s summation rule is exact for the ideal-gas model. At high pressures or near condensation, intermolecular forces and finite molecular volumes become important. Partial pressure then no longer directly equals the thermodynamic driving force; fugacity corrects pressure in chemical potentials. For many school and laboratory calculations at moderate conditions, Dalton remains a good approximation. State the ideal-gas assumption when precision or extreme conditions matter. Check whether the mixture is dry or humid and whether all components share the same available volume. At high pressures, an ideal partial-pressure model may no longer describe real interactions accurately.