Do you give your students steps to follow when working empirical formula problems? I find that it REALLY helps my students.
But, sometimes it is hard to come up with exactly the right steps, so I’ve laid them out for you. Just copy them down or pin this post so you can share them with your students.
Let’s get started.
#1 Get to Grams
Usually you are either give your students the grams of each element or give them the percentages of each element within a compound. Convert percent to grams by dividing, but save yourself teaching headache by just telling them to exchange the % to a gram sign. Do make sure they understand that we are assuming 100 grams of substance, which is why that trick works.
When you begin to teach these problems to your students, start with problems that require them to convert percentages to grams as opposed to the problems that start with grams. That way, in problems students don't have to convert to grams, they feel like they get to "skip" a step. If you teach it the other way around, students feel like you are giving them extra work.
It's all in how you present it.
#2 Set Up Converting to Moles
Now students are in grams, tell them that the second step is to set up converting to moles. To do that, they need to divide the grams by the atomic mass of the element.
Tell your students specifically that they will have to look the number in the denominator up by finding it in the periodic table. Otherwise, they will ask you 1600 times. In one day.
At this point, stop and ask if any student has a question.
#3 Divide by Atomic Mass
For example, tell your students that they have 62.1 g C, and they need to divide it by 12.01 g/mol. Tell them to write that new number down to three decimals. Repeat that for all the elements in the problem.
Tell your students not to round until the end of the problem. That will keep all of your students at the same place with the same numbers.
#4 Divide by Smallest #
Tell students at this point they will have 2 or 3 or 4 awkward looking numbers and that is ok. Tell them to pick the smallest awkward number and divide all the by that number.
For example, looking at 5.170, 13.663, and 1.720, 1.720 is the smallest number. Divide 5.170, 13.663, and 1.720 by the 1.720.
Tell students to write that new resulting number down.
#5 Use those Numbers as Formula Subscripts
The last step students did should get them a whole number. That is the whole point of the division. Tell students that if they got 2.001 go with 2. Tell students we must have whole numbers.
You may also want to mention to your students if they end up with a 2.5 they need to either 1) check their work or 2) multiply all the numbers by 2 to make it even.
Bonus Step for Molecular Formulas
Tell students if they need to find the molecular formula, find the molar mass of their current formula, and divide by the molar mass given in the problem.
Empirical Steps Made Easy
Your students are going to love that you gave them these steps!
Now when you put your examples on the board, make sure you do one example where you do the work and write the steps right next to the work. Even better, color code the steps to the work.
If you liked these steps, you are going to love this illustrated guide to empirical formulas where you will make this process super easy for students to learn. Molecule Men who are working in an empirical formula factory use a step by step process to get the right formula out.
Hi! I'm CoScine. I write chemistry worksheets for visual learners. They are fun, easy to follow, and most of them are quick to grade. Since I started my teaching career at the college level, these are just simple chemistry. These worksheets are hard core science.