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.


Supporting Information: 


Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models.


Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models. Plan and conduct an investigation individually and collaboratively to produce data to serve as the basis for evidence, and in the design: decide on types, how much, and accuracy of data needed to produce reliable measurements and consider limitations on the precision of the data (e.g., number of trials, cost, risk, time), and refine the design accordingly.

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Comments 6

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 added a procedure handout for the old version to the post. You should see it under Supporting Information. I have not written up the new procedure for student use yet.






MeiPing Yang's picture
MeiPing Yang | Sun, 12/17/2017 - 13:02

This is a wonderful idea.  I had to cut down my piece of Mg due to time constraint.  Now I can use a bigger piece and even less time for reaction to happen.  Will definitely try this year.

Eric Pantano's picture
Eric Pantano | Tue, 08/06/2019 - 08:36

We use a one-hole stopper and tie the Mg piece with regular old string. Trap the end of the string between the eudiometer and the stopper, and invert.  Does a pretty decent job.

Aaron Burg's picture
Aaron Burg | Fri, 08/19/2022 - 07:53

I've done this lab for a number of years and it just occurred to me this year that the gas is a mixture and that should have implications on volume. The moles of H2 and the moles of water as to cause the total volume. Is this correct reasoning?

My question is this... All though small, should we be correcting for the moles of H2 gas only?

I'd think the Pbar-PH2O would provide PH2. And then pressure fraction is equivalent to mole fraction which can be multiplied into the volume of gas in the gas collecting tube.


Each year we have error in this lab. An error that seems like it should be correctable. I had one group try this correction and the data were close to spot on. Coincidence or has this been omitted from the lab because it doesn't matter or is my reasoning incorrect?

Michael Morgan's picture
Michael Morgan | Sat, 08/20/2022 - 09:39


I agree that the water vapor makes a difference. I typically have the students measure the temperature in the water bath and look up the vapor pressure of water at that temperature. They can subtract that from total pressure and get the pressure of the dry H2. It seems to bring the calculations really close to acceptable values. So yes your should do the correction. I am shocked at just how close to literature values my students get out of this data.