mmHg and mH2O are units for pressure, but what is the volume of the Hg and H2O per unit of mmHg and mH2O? I feel like it should be area multiplied by height, instead of just length such as "mm" and "m" as in the case of mmHg and mH2O. Is there some pre-decided unit of area that is the standard to be multiplied by these units of length (mm, m)?

Question

mmHg and mH2O are units for pressure, but what is the volume of the Hg and H2O per unit of mmHg and mH2O?  I feel like it should be area multiplied by height, instead of just length such as "mm" and "m" as in the case of mmHg and mH2O.  Is there some pre-decided unit of area that is the standard to be multiplied by these units of length (mm, m)?

kainui · Answer

So mmHg literally means millimeters of mercury. The height of the mercury is measured in a manometer when two different pressures are pushing on it on either side. I can derive the formula for you if you're interested, but what's important to know is that the area cancels out in the end, so that it doesn't matter how thick our manometer tube is as long as it's the same area throughout. 

So if you wanted the volume of mercury, well then you could measure the area of the manometer and multiply that with the height to get volume. If you look up the density of mercury you can find out how much that column of mercury weighs, and the force it exerts is the same as the force of the gas pushing on the other side, and that's why they're in equilibrium.

anonymous · Answer

thank you :D

kainui · Answer

Yeah, I'm taking physical chemistry 1 right now and it's interesting stuff, we got to derive a formula for how pressure decreases exponentially as you go up higher in the atmosphere. The interesting thing is that this formula includes the force of gravity in it, so if you're able to find out the mass of, say, Jupiter you can find out the pressure of gas at any height in a distant planet's atmosphere too!