In chemistry class today, we learned how to determine atmospheric pressure using a mercury barometer, and the pressure of a confined gas using a mercury manometer. I understood how to used the equations fine, but I did not know where they came from. I thought it would be a relatively simple matter of deriving them (save the constants) but the more I thought about it the more confusing the problem became. In class we were told that the pressure came from air being forced downwards due to gravity. Makes sense, I figure. However, in the case of a manometer, The amount of gas pushing down on the mecury in the tube would change based on the size of the spherical container that the gas is contained (if the gas is the same density) More volume --> More mass --> More force pushing down on the mercury Pressure = Force/Area There is now MORE force, but the same amount of area of exposure from the gas to the tube. If we go with this gravity theory, then pressure becomes arbitrary. It seems as though heat should be involved too. More heat --> More kinetic energy --> More pressure In other words, I don't understand how gas pressure works. Intuitively, it seems to be a weird mix of gravity, heat, and properties of the container. My question is this: If you know volume, density, mass, acceleration due to gravity, surface area of container, and the temperature of a gas, how do you calculate gas pressure, and how do you derive that equation?