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combined gas law

2/5/2014

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The past several posts have been about gas laws.  The Combined Gas Law does just what it says, the 3 gas laws combine as follows:

P V / T  = P V / T

Again, temperature must be in Kelvins.  This ensures that negative values and zero are not in the denominator of the fraction.

What is nice is that all 3 gas laws are neatly in this relationship -- all you have to do is eliminate (cancel) the constant variable.  It's now worth it for a student to memorize because this relationship contains a lot of information.

As I taught my students, I would cover the T's to get PV = PV, cover the P's to get V / T = V / T , and cover the V's to get P / T = P / T.  Very useful!

A very typical mathematical problem is to convert the volume of a gas sample with values of pressure and temperature to a volume at standard conditions of 1 atmosphere and 273 Kelvin (0 degrees Celsius).

You can algebraically rearrange the variables and then plug in the values for a solution; however, it is six variables and can get a little confusing.  Plugging all variables with units and then solving algebraically is a very reasonsable method.

Here is a worksheet with solutions:
http://misterguch.brinkster.net/WKS001_007_146637.pdf

This video contains a discussion of molar volume of a gas, but then several demonstrations.  It is longer (15 minutes), so you might want to skip ahead at times.  I recommend wearing eye safety goggles while working with any of these demonstrations.  I tend to err on the side of caution!



Check out my lab book "Chemistry on a Budget"  at:
http://www.amazon.com/Chemistry-Budget-Marjorie-R-Heesemann/dp/0578129159/ref=sr_1_1?s=books&ie=UTF8&qid=1389410170&sr=1-1&keywords=chemistry+on+a+budget

It contains a lab that collects a gas without a chemical reaction!

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!

*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!
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    Marjorie R. Heesemann is a chemistry teacher with 15 years of experience who is now working to develop resources for the Chemistry classroom.

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