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Chemistry Lab: Percent Composition

And students get to see a hands-on application of the problems we solve in the classroom.

For the extremely busy teacher, it doesn't get much better than this for a great chemistry lab.  When the materials list consists of.....

Baking soda, 2N sulfuric acid, a spot plate, a Beral pipet ( or any dropper), a balance, and a test tube

........ it is a good day for the chemistry teacher!!

The percentage composition of a compound is the percentage by mass of each of the elements in the compound.  The purpose of this experiment is to determine the percent composition of carbon in a sample of sodium bicarbonate, NaHCO3.  
When sodium bicarbonate reacts with sulfuric acid, carbon will be released in the form of carbon dioxide.  According to the Law of Definite Proportions, the mass percentage of carbon in the bicarbonate will be constant, no matter how much sodium bicarbonate is used in the experiment.

Students simply weigh out an amount of baking soda. Drops of 2N sulfuric acid are added to the baking soda, one drop at a time.  The reaction releases carbon dioxide. Students continue adding drops of acid until there is no further reaction.  
The mass of the apparatus is taken both before and after the completion of the reaction to determine the amount of carbon dioxide that has been released. From the mass of carbon dioxide released, students can mathematically determine the amount of carbon in the released sample, and therefore the percent composition of carbon in sodium hydrogen carbonate.

The student will determine the percent composition of carbon in sodium hydrogen carbonate, both experimentally and theoretically.  The student will then determine his/her percent error.

What are the "pros" to this lab?
  • It provides hands-on practice in determining the percent composition.
  • It reinforces the calculations that are being taught in the classroom.
  • Simple materials list.
  • Easy set-up and easy clean-up.
  • Allows students to compare a "true value" to their experimental data.
  • Student loves watching the bubbling in the test tube!!  :)
  • No real safety issues since a fairly dilute solution of sulfuric acid is being used.  (Students MUST wear safety goggles!)
What are the "cons" to doing this lab?  None that I can think of!

You can find this lab in my TpT store by clicking this link:  The Percent Composition of Carbon in Baking Soda.

Have fun teaching!


  1. Hello! I came across your lab and I was trying to figure out how the calculations work! You can take the weight of the starting baking soda, well plate, and test tube. You can weigh the final products (water, sodium sulfate), well plate, and test tube. It is not possible to know how much sulfuric acid was added in grams unless you keep track of that separately, as the sulfuric acid is reacted and the carbon dioxide gas is lost. Am I missing something here?

    1. The sulfuric acid is in the test tube. You weigh the sulfuric acid in the test tube along with the plate and the baking soda. The sulfuric acid that you add to the baking soda comes from the test tube; therefore, the only mass that is lost is from the carbon dioxide escaping. The difference in mass before and after the reaction is due to the carbon dioxide that escapes. There is no need to worry the sulfuric acid. I hope this helps.