# What does gases at constant pressure mean?

My teacher told me that if we keep some weight on a piston containing 2 different gases, both gases will be at constant pressure, and hence the volume occupied by each gas will be proportional to number of moles of each gas according to the ideal gas law ($pV=nRT$)

My doubt is that won't the 2 gases exert different pressures and the sum of the two different pressures will be equal and opposite to the pressure exerted by the weight? Also, if the two gases exert different pressures, then how can we say that both gases are at constant pressure? Do gases at constant pressure exert equal pressure?

Without a clearer setup it's difficult to say whether someone misspoke, whether you misunderstood precisely what your teacher was saying, or whether your teacher was incorrect.

If the setup is that there is a single piston that contains two gases, and a weight is placed upon it, then:

1. The volume of each of the gases in that piston is equal to the entire volume of the piston. The volume of any gas is the entire volume of its container. If the gases are ideal, then the volume of the piston will be the sum of the volumes of the piston if each of the gases were kept in it individually.
2. The pressures of the two gases sum to the pressure exerted on the piston by the weight (and the outside atmosphere.) The pressures will be constant as long as the amount of gas present, the applied pressure, and the temperature are kept constant.

If the setup is that there are two pistons that each contain a gas, and an identical weight is placed upon each (or a single piston filled with two gases, one at a time) then:

1. The volume of the two pistons will be proportional to the number of moles of ideal gas that it contains, as long as both are also at the same temperature (we're already assuming the same pressure due to the identical weight.)
2. The pressures of the gases in the two pistons will be equal, because they are opposing the same weight + outside atmospheric pressure.