Understanding Pressure

Concept: The different units for Pressure

Video Transcript

Despite the fact that the SI unit for pressure is Pascal, chemist all around the world continue to use different units for pressure, because they're just accustomed to it. The Pascal is the SI unit for pressure, but most of the time, you'll see in your books and in different types of research journals from chemists and scientists, they instead like to use atmospheres, torrs or millimeters of mercury. Sometimes you might see bars as well. The Pascal is the SI unit but they tend to like to use these other types of units for pressure.
Now, here we have the different names for pressure. All of these are just different types of pressure, basically different units for it. They're all saying the same thing. An atmosphere is a unit of pressure just like a torr is, just like a bar is, just like kilopascal is. It's going to be important that you know how to convert from one to the other.
It's a long list, but the ones that professor usually focus on the most are: atmospheres, millimeters of mercury, torrs and bars. Those are usually the ones that we see the most, and, of course, Pascals.
Just realize all of them are equal to one another, so we can say that 760 torrs are equal to 1.01325 bars. You could say that one atmosphere is equal to this many Pascals. All these units are in agreement with one another. They all equal one another. We're going to say that one atmosphere equals 760 millimeters of mercury or 760 torrs.
What I also want you to see is that just look at these two right here, the millimeters of mercury and the torrs, they both have the same number, so 760 millimeters of mercury equals 760 torrs. What this really means is that for every one millimeter -- what this means is for every one millimeter of mercury, we have one torr. They're basically the same units.
Just remember that what the SI units for pressure are, what are the SI units for force, what is pressure defined as and then the connections between the different units for pressure because when it comes to gases, we have to remember this because pressure plays a very large role in the property that a gas has. 

Although the SI unit for pressure is the Pascal, most professors use atm, mmHg or torrs as the everyday units for pressure. 

The conversion of units of pressure may sometimes be required in order to answer a quantitative question. 

Example: A geochemist heats a limestone (CaCO3) sample and collects the CO2 released in an evacuated flask. The CO2 pressure is 278.1 mmHg. Calculate the CO2 pressure in torrs and atmospheres.


The conversion of units of pressure is simply a dimensional analysis question. Recall the dimensional analysis relationships been volume and length.  

Problem: If the barometer in a laboratory reads 34.2 inHg what is the pressure in bars? (1 in = 2.54 cm)