Can someone explain Partial Pressures?

Let's start with Dalton's Law. Each gas in a mixture of gases has a partial pressure, and the total pressure is the sum of all of them. Let's say we're talking about air w/ zero humidity (to keep it simple). Atmospheric pressure is 760 mm Hg, and air is 21% oxygen and (roughly) 79% nitrogen. So the partial pressure of oxygen in the air is 21% of 760, or 160 mm Hg. The partial pressure of nitrogen in the air is 79% of 760, or 600 mm Hg. In reality, there are small quantities of other things in the air, but if you knew their percentages, you could calculate their partial pressures, and you could add them all up together to get the total pressure. If you are talking about humidified air, you also have the contribution of the partial pressure of the water vapor that's in the air.

Now let's talk about Henry's Law. Say you have the dry air mixture we talked about above--the partial pressure of oxygen is 160 mm and the partial pressure of nitrogen is 600 mm Hg. We put that mixture in contact with a liquid, and some of the oxygen and some of the nitrogen will dissolve in the liquid. How much depends on a bunch of things, including what the liquid is and the temperature. But the amount of oxygen that dissolves will be proportional to its partial pressure in the gas mixture, and the amount of nitrogen that dissolves will be proportional to its partial pressure in the gas mixture. So if you doubled the partial pressure of oxygen in the gas mixture, twice as much would dissolve in the liquid. If you cut the partial pressure of oxygen in half, only half as much would dissolve in the liquid.

wow, that helped me so much! you explained it perfectly!

here is a question on my study guide i dont understand:

Scientifically how do we depict partial pressures?

does that just mean what do you multiply to get them?

don't know what they mean by 'depict'. They are reported in the same units as total pressure, perhaps that's what they mean? They will vary with conditions, in nursing we typically deal with body temp (37 C), could be something like that--maybe others have better insight.