# Chemistry

Help for teaching molecular formulas

I just gave a test on calculating empirical and molecular formulas to my 11th grade chemistry class. Even after all of the practice problems that we worked together in class, it seemed as if most of my students were clueless. Frankly, I'm stumped. Does anyone have suggestions for reteaching and/or teaching it better the first time? Thanks!

Kimette Witt
455 Activity Points

Eric Carlson
30030 Activity Points

Gisela Dumm
3745 Activity Points

Empirical and molecular formulas definitely comprise a conceptually difficult topic for high school students to understand. Here is a strategy that may make at least the mathematical process more accessible. On a graphing calculator, make use of the "Lists" function. In L1 input the grams of each element present (e.g. for water, entering 11% hydrogen and 89% oxygen would be 11 g and 89 g). In L2 input the molar mass of each of those elements (1.008 and 16.00). At the very top of L3 input "L1/L2" and the calculator will automatically compute the ratio of the elements present. Then you can further use this function to divide by the smallest amount in L4 (or do it by hand). This is hard to explain in words but I promise that students who have some experience with graphing calculators (like if they use them a lot in math class) will easily be able to do the steps above. (An alternative would be to do all of this in an Excel spreadsheet.) This method doesn't quite address the rationale behind the concept, but it does make the computation a whole lot faster! I've taught this strategy to my AP Chemistry students and it definitely helps them eliminate silly mathematical errors since everything is organized by function in the calculator itself.

Francesca DePasquale
680 Activity Points

http://www.files.chem.vt.edu/RVGS/ACT/notes/The_Mole.html I really like this approach http://www.ausetute.com.au/empirical.html

Pamela Auburn
68515 Activity Points

empirical and molecular formulas are difficult for students to grasp initially because, in the classroom, it's pretty hard to envision. I use foods with distinct and easily separated parts for a hands-on visual. I can't find my newest one for some reason...must be on a flash drive or another computer at school, but you can check this one out to get an idea

Attachments

bw-oreo.doc (0.02 Mb)

Garrett Arakawa
600 Activity Points

Here is a ice crime scene who done it that incorporates empirical forumlas. It might uses as an end of the unit assessment http://filebox.vt.edu/users/slwood06/portfolio/DocumentsForLinks/moleunit/EmpiricalMolecularFormulaLesson.htm

Pamela Auburn
68515 Activity Points

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