PV = nRT where P = pressure
V = volume
n = number of moles of the gas
T = temperature (in Kelvins! No negatives or zeros allowed!)
And R is Ideal Gas Constant. What's that? I used to tell my students that we don't have the perfect way to measure everything, so R is an adjustment factor that makes this relationship work. That's true for lot of mathematical constants.
So R can be represented with different units -- and the units have to cancel -- you can't go on autopilot with this, and any, math. One example of R is:
R = 0.0821 L x atm/ mol x K
Students may have to convert values from one unit to another. This is where factor analysis is really handy. For example:
730 torr x 1 atm/760 torr = 0.96 torr
If you're worried about significant figures, there are two significant figures in the initial value and two in the answer, and the conversion factor is not considered to be limiting. I'll examine significant figures more in depth in a different post.
These problems are pretty classic, sort of the 10 point problem at the end of the test because the problem requires a lot of math to solve. Here's an example:
The 2nd example in this video also has a nice twist of requiring that the answer be in Celsius so you have to convert the Kelvin answer to Celsius as well.
Now an ideal gas stays a gas no matter what you do to it, but we live in the real world, and the attractive forces of gas particles do take over and gases do form liquids and solids. So, the classic test question is to ask under what conditions a gas acts like an ideal gas and what conditions for a real gas.
An ideal gas won't ever liquefy, so conditions of high temperature, low pressure, and low particle mass (such as Hydrogen at 2 g/mole and Helium at 4 g/mole) would lead to low attractive forces and the substance would stay a gas.
Conditions where the particles would be closer together and attractive forces would "kick in" would be low temperature, high pressure, and high particle mass (such as CH4 or CO2). Here's quick video with a clear list of the characteristics:
A lab that uses the ideal gas law is reacting magnesium ribbon with hydrochloric acid -- this is an example of a single-replacement reaction, but here's a video example:
It's nice combination of stoichiometry and gas laws -- a very typical lab performed in first-year chemistry. I hope there are eudiometer tubes in your department! Ask if you don't know where they are!
Here's one procedure:
Here's another lab handout with nice illustrations:
You can go online to get the daily barometric pressure (vs. the LabPro sensor used in Step 1):
Enter your zip code to get local weather readings.
The barometer reading is in inches of Hg, so it will have to be converted. For example:
30.33 in Hg x 1 atm / 29.92 in Hg = 1.014 atm
Check out my lab book "Chemistry on a Budget" at:
Each lab is presented with two possible report formats -- both with the same procedure -- one with 10 questions to be answered as a conclusion, the other with a full laboratory report required. This was to give the teacher the option of what type of report is desired!
This book contains a lab that collects a gas without a chemical reaction, which is handy if you don't have the eudiometer tubes for this experiment.
*I'd love to hear from you about your experiences, your questions or if you have ideas for other topics for this blog.
Have a good week!