Thursday, February 19, 2015

The Lighter Molar Mass Lab

OK.  First things first.  You get some very funny looks when you buy this many lighters at Giant Eagle.  But, if you can push past that, and the occasional derogatory remarks, you have the beginnings of one of the very first inquiry labs I ever tried.

I teach a lesson on the Ideal Gas Law, followed by a lesson on Dalton's Law of Partial pressures.  As part of my Dalton's lesson, I do a demo where I collect the CO2 from a diet coke by water displacement so that kids see why we need to learn about Dalton's Law.  Both lessons are followed by your standard practice problems, including one where we calculate molar mass of a gas.

The next day in the lab I talk briefly about lighters and butane.  I suggest that if we collect a sample of the butane, by taking it out of the lighter, we can make some measurements and calculate its molar mass.  With just that short intro, I send them into the lab to develop a plan.  I won't give them the lighter until they tell me how they will measure all the data they will need.

What I know about kids, having done this lab for many years, is that they don't really know what they need.  And, given the chance, they would just grab the lighter and start collecting gas, only to find out later that they missed a step or a measurement.  They pretty quickly realize that they will apply Dalton's Law and the Ideal Gas Law, use the barometer for the room pressure, a thermometer for the temperature, a flask or something to collect the gas and measure the volume.  The question I ask over and over the day we do this, though, is "how will you measure the mass of the gas?"

Often students say that they will measure the mass of the flask with the water in it and then measure it at the end when it is full with gas.  This idea of massing before and after is pretty standard at this point in the year, but it is tricky when you're dealing with a gas.  Gas goes into the flask and water comes out of the flask, so subtracting the two masses doesn't work.  It's a head scratcher for them.
It's hard to see the lighters in these pictures I am including, but they are there, under the water, beneath the mouth of the flask.  While in this position, students ask great questions:  
  • How much gas should we collect?  
  • Do we need to empty the lighter?  
  • How will we know when its empty?  
  • Do we take the temperature of the air or the water?  
My answers are usually more questions:  
  • How much do you think is a good-sized sample?  
  • Do you need to empty the lighter?  
  • Which temperature is the one that more closely approximates the temperature of the gas you collected?  
  • Isn't it great when you get to make all the decisions about the experiment? 
The results of the experiment come out ok.  Most students calculate the mass to be somewhere between 45 and 80 g/mol; the real value is 58 g/mol.  I like that they have some error because then they also get the opportunity to scrutinize their procedures to consider where they they could make improvements.  The reports are usually easy to write because the students had to do so much thinking BEFORE and DURING the lab.  Sometimes students even race to calculate the molar mass while they are still in class to see if they got close.  Any day where students are anxious to see if their results are good feels like a good day to me. 

I am adding this description to my page of inquiry-based chemistry labs.  If you like this one, check out some of the others.    


  1. Silly question: At the end of the lab, how do you dispose of all the butane? Fkare it off from the flasks? And what do you do with all the not quite empty lighters?

  2. It's not that much butane. When the kids tip the flasks upright, the butane is released into the classroom. The lighters last several years, so, when the lab is finished, I put them all back into the bag and store them for next year.