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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?
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I'd like to say I have a definitive answer for you, but I've only had moderate success with this myself. Here are some things that I have found helpful in getting the concept into the kids heads.
First, make sure they have a good grasp of what an empirical formula actually is. I was surprised at first at how many calculations kids can do with no conceptual basis of what it is they are calculating. But many kids just find the step by step procedure that the book so kindly provides, and follow it.
So ask them, what is the empirical formula of sodium chloride? of hydrogen peroxide? of the hydroxide ion? of methane? of ethane? of nitrogen dioxide? of dinitrogen tetroxide? of carbon monoxide?
Give them a list of formulas, it can be completely random, and ask which of those could, and which could not be empirical formulas. Hopefully they will note that if subscripts have a common factor, you don't have an empirical formula
Second, give some context. Why do we care about empirical formulas? What value do they have in chemistry? I like to frame it this way: put out a chemical, could be water, could be sodium chloride, could be ethanol, and ask them, "If you were given the assignment to figure out what atoms were in this chemical and how many of each kind there were, what information would be helpful?" It may take some prompting, but they should see that percent composition is quite helpful here.
(As a brief aside, your inquisitive students might ask, "How can we determine percent composition?" Alas, such students tend to be far and few between. But it's an utterly fantastic question. If I handed you a mass of 10 grams of sodium chloride and asked you to experimentally determine how many grams of chlorine were in it, what would you do? Does the book even remotely address this? Most don't. Percent compositions are usually handed out like revelations from on high.)
Finally, what I have found most helpful is to make analogous problems at a macroscopic scale. For example,
"At Standard Weight High School, all freshman weigh 100 pounds, all sophomores weigh 125 pounds, all juniors weigh 150 pounds, and all seniors weight 175 pounds. A group containing only freshman and juniors is found to be 40% freshman and 60% junior by mass.
What's the ratio of freshman to juniors?
If the group weighs 500 pounds all together, how many freshman and juniors are in the group?"
Because they can conceptualize the pieces, such problems aren't as daunting. Once they get comfortable with problems like these, point out that they are identical in every significant way to empirical and molecular formula problems. The only difference is the mass of the individual pieces.
I hope this proves helpful for you.
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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.
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I really like this approach
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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
bw-oreo.doc (0.02 Mb)
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Here is a ice crime scene who done it that incorporates empirical forumlas. It might uses as an end of the unit assessment
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