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 these containers, we have these blue dots which represent gas molecules. 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 meters squared. So pressure itself is just a force that the gases exert over an area. So here we're gonna say that pressure equals force over 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 then just torr. These units for pressure have their own pressure value which can be related to one another. 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 kilopascals. 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. These numbers relate to one another. We said that they're a way to connect to each other. That means that one atmosphere equals 760 millimeters of mercury, or 1.01325 bars equals 14.696 psi. 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 states that the pressure in Denver, Colorado, which has an elevation of 5,280 feet, averages about 24.9 inches of mercury. Now, we need to convert this pressure into millimeters of mercury and atmospheres. This involves a dimensional analysis, where we convert from inches of mercury to millimeters of mercury and then to atmospheres. Let's begin with 24 inches of mercury. We want to eliminate inches of mercury, so we place inches of mercury in the denominator.

From dimensional analysis, there's a known relationship between inches and centimeters. We'll convert from inches of mercury to centimeters of mercury. If you recall from my dimensional analysis videos, 1 inch is equal to 2.54 centimeters. With inches of mercury cancelled out, we now have centimeters of mercury. Next, to convert centimeters of mercury to millimeters, we use the fact that 1 centimeter equals 10 millimeters. After cancelling out the relevant units, we are left with millimeters of mercury, which calculates to 632.46 millimeters of mercury. Given that the values within the question have three significant figures (24.9), we'll round this to 632 millimeters of mercury in three significant figures.

Moving forward to convert the 632.46 millimeters of mercury into atmospheres, we use the conversion factor where 1 atmosphere is equal to 760 millimeters of mercury. This conversion results in approximately 0.832 atmospheres as the final answer. So, just approach this like any dimensional analysis type of question to convert from one pressure unit to another.