What is the Dalton's law of partial pressures?
Dalton's law of partial pressures says that the total pressure in a container will be equal to the sum of the individual pressures of the gases present in the container.
If a container is filled with butane and water vapor. The total pressure of gas in the container will be equal to the pressure of butane and the pressure of water vapor added together.
If the pressure of butane is 0.85atm and the pressure of water vapor is 0.15atm, the total pressure will be calculated as follows:
0.85atm + 0.15atm = 1atm
Here is a video which discusses another example of using Dalton's law.