Dalton's Partial Pressure Calculator

The law of partial pressures states that the total pressure of a mixture of non-reacting gases equals the sum of the partial pressures each gas would exert if it occupied the container alone.

The partial pressure of a gas equals its mole fraction, its share of the total moles present, multiplied by the total pressure of the mixture.

The mole fraction of a gas is the moles of that gas divided by the total moles of all gases in the mixture, a value between 0 and 1. In a mixture of 2 mol O2 and 3 mol N2, the mole fraction of O2 is 2/5 = 0.4. Multiplying the mole fraction by the total pressure gives that gas's partial pressure, and all mole fractions in a mixture add up to 1.

When you collect a gas by bubbling it into an inverted container filled with water, the gas mixes with water vapor, so the measured total pressure includes both. By Dalton's law the pressure of the dry gas is the total pressure minus the vapor pressure of water at that temperature: P(gas) = P(total) - P(water). At 25 C the vapor pressure of water is about 23.8 mmHg (3.17 kPa), so you subtract that value to get the true partial pressure of the gas collected. A vapor-pressure table gives the figure to subtract at each temperature.

Yes. Rearranging Dalton's law, a gas's mole fraction equals its partial pressure divided by the total pressure, and multiplying that fraction by the total moles gives the moles of that gas. For example, a gas with a partial pressure of 4 atm in a 10 atm mixture containing 5 mol overall has a mole fraction of 4/10 = 0.4 and accounts for 0.4 x 5 = 2 mol. You can also get the moles directly from the ideal gas law, n = PV/RT, using the partial pressure as P.