Pressure Units - Online Tutor, Practice Problems & Exam Prep

Pressure Units

The SI unit for pressure is the Pascal, abbreviated as PA, and it's named after the French mathematician, Blaise Pascal. When it comes to pressure, let's think about what that really means. When it comes to gases, they can be seen as a collection of molecules that move in random directions in straight lines. So here we have an image of a container. Inside of these containers, we have these blue dots which represent gas molecules. All they're doing is bouncing around randomly, bumping into the sides of the container, bumping into each other. How does this apply to pressure? Well, pressure is the force that a gas exerts on the wall of its container. Connected to pressure are two ideas: F, which equals force in SI units of newtons, and A which represents area, and its SI unit is in square meters. So, pressure itself is just a force that the gases exert over an area.

P = F A

This idea of gases existing within containers is going to be delved deeper into when we talk about ideal gases and eventually non-ideal gases.

Pressure Units

Additional non SI units for pressure used by chemists are atmospheres, which is ATM, millimeters of mercury, which is mmHg, and Torr. Now, these units for pressure have their own pressure value which can be related to one another. Now remember, if you see a purple box, that means you have to memorize it because usually, it's not given to you in terms of a quiz or exam. The pressure values associated with atmospheres, millimeters of mercury, and Torr are 1760 and then 760 again.

The less commonly known ones, pascals, kilopascals, bars, and psi aren't in purple boxes, meaning they're not usually expected to be memorized. They'll be given to you in some way within the question or formula sheet. Now their pressure values are 1.01325×105 kPa. Remember kilo is 103. So, this is 101.325. Bars here is 1.01325. And then finally, if we look at psi, psi is 14.696.

Now, these numbers relate to one another. We said that they're a way to relate to each other. That means that one atmosphere equals 760 millimeters of mercury, or 1.01325 bars equals, 14.696 psi. So these are just ways of them connecting to one another. But again, it's the first three that are going to be the most important. Even though they're non SI units, they're the ones most commonly used by chemists.

So now that we know these different types of pressure values, let's continue onward to an example.

Pressure Units Example 2

In this example question, it says the pressure in Denver, Colorado, which has an elevation of 5,280 feet, averages about 24.9 inches of mercury. Now we have to convert this pressure into millimeters of mercury and atmospheres. Alright. So this is basically a dimensional analysis type of question; we have to convert from inches of mercury to millimeters of mercury. Once we have that, we can then change those millimeters of mercury into atmospheres. So let's start out with 24.9 inches of mercury. We want to get rid of inches of mercury, so we're going to put inches of mercury on the bottom. And then remember that from dimensional analysis, we know there's a relationship between inches and centimeters. So we're going to go from inches of mercury to centimeters of mercury. Now recall, if you've looked at my dimensional analysis videos, you know that 1 inch is equal to 2.54 centimeters. Here our inches of mercury cancel out, and now we have centimeters of mercury. Now we want to get rid of centimeters of mercury, so put it on the bottom, and we can go from centimeters to millimeters, and we can say here that 1 centimeter is 10 millimeters. Casting out everything, what we'll have left at the end is millimeters of mercury, which comes out to 632.46 millimeters of mercury. Now because our values within the question have 3 significant figures, with 24.9 not 24.9, and 3 significant figures here as well. We're going to put this in 3 significant figures. So 632 millimeters of mercury. Now we want atmospheres, so let's take that 632.46 millimeters of mercury and we're going to convert that into atmospheres. So remember here, 1 atmosphere is equal to 760 millimeters of mercury. So when we do that, we're going to get as our answer 0.832 atmospheres as our final answer. So just approach this like any dimensional analysis type of question to convert from one pressure unit to another.