Molar Volume of Gas - Lab Tip

Molar Volume of Hydrogen Gas Lab

If you read my last blog post you heard discussion of putting together a practical and useful professional development opportunity for chemistry teachers. It is a conference for teachers, by teachers. My goal is for every presenter to bring in one idea that they find useful in their classroom; One good demo, one good lab, one good original idea about how to do something that we are all doing to begin with.



Figure 1 - Magnesium metal inside a copper wire cage.


While working on the schedule I came across a presentation from three years ago that was absolutely fabulous and truly had an impact on my classroom. It reminded me of why we really need to have professional development presented by classroom teachers who are practicing what they preach every day. Many of us are familiar with the Molar Volume of Hydrogen Gas lab. It was on the required lab list for AP Chemistry for many years and I can find versions of it that date all the way back to Chem Study in 1962. Every version seems to take a strip of magnesium and put it into a small cage of copper wire and insert it into a gas collection tube (Figure 1). Six molar HCl is then reacted with the magnesium. A little twist to this procedure was suggested by an amazing teacher, Werner Willoughby, formerly of San Pedro High School in Los Angeles. He presented the idea of using 12 M HCl (only handled by the instructor) and instead of wrapping the magnesium into the cage of copper wire to simply fold it over and place it in the gas collection tube and let friction keep it in place (Figure 2). Since the acid is so concentrated it reacts almost instantly and provides two great benefits. One is that it does not require a cage of copper that may or may not keep the magnesium in place. Second is that it reacts so quickly it allows for multiple trials in one class period. This saves me a great deal of time! I run after school labs so that I can have a two-hour block of time. I have to do this twice a week to accommodate all the students enrolled in my class. Now I have one less week that I have to stay after school and force the students to stay after school.



Figure 2 - Magnesium metal held in place by friction.


Now I don’t want you to think that the purpose of this blog post is only to describe this technique because it is not. The main purpose is to point out how important continuing education is for teachers and the importance of it being provided by other teachers. I am very confident that all of you reading this have at least one trick you have come up with that I have not thought about. Why not get out there and attend an AACT, BCCE, Biennial, ChemEd or NSTA style meeting and help share your passion and your expertise with others. Every year I organize this meeting I come home with at least one great idea. I have never failed to learn something at every major conference I attend.


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Comments 2

Chad Husting's picture
Chad Husting | Sun, 10/15/2017 - 19:13

Looks really cool....can you elaborate how you would physically set this up and deliver the acid?  Thanks.

Michael Morgan's picture
Michael Morgan | Mon, 10/16/2017 - 09:18

Hi Chad,

I have the students bring the gas collection tube to me at the fume hood and I pour in the acid. They fill to the top with water and insert the magnesium ribbon and then a two hole rubber stopper. Pretty much the same way you normally would do the lab with 6 M HCl and a copper cage. I will put a link to my procedure hadnout for the old version below. I have not written up the new procedure use for student use yet.