Dalton's Law

The pressure of a mixture of gasses is equal to the sum of the pressures of the different components of the mixture.

$P_{total} = P_A + P_B + P_C + ...$

The partial pressure of each gas is the amount of the listed gas particles present divided by the total amount of particles. Both of these values are in moles. The total pressure is the sum of the partial pressures.

$P_A = P_{total} * X_A$, where $X_A = \frac{\text{moles } A}{\text{total moles}}$

Water displacement uses Dalton's law to get the amount of gas collected based off the amount of water displaced. Be sure to note the water vapor pressure and subtract it from your values before calculating. You can determine the water vapor pressure using a lookup table based off the current temperature.

$P_\it{\text{gas collected}} = P_{total} - P_{H_2O}$