Lesson 4 of 9
Objective: SWBAT calculate molar mass and convert from grams to moles based on real world nutrition labels.
One of the major headaches in teaching stoichiometry is that it can be so boring because it is so math-heavy. The numbers take over and the kids forget about the chemicals. I wanted to bring some real world context to the work with the mole, so I thought, "We can do moles of chemicals based on nutrition labels!"
Which sounds awesome, until you're at the grocery store buying beverages for 120 kids, and then it sounds a little crazy. Especially when two days previously, one student was talking about how she doesn't drink pop, which is what we call it here in Chicago. (As a Chicagoan, it kills me to call this lesson "Stoichiometry of Soda" but I couldn't resist the alliteration). Others are lactose intolerant, the athletes are in season and trying to be careful about what they eat and drink, and my two expectant mothers need to eat healthy.
So for this, I mixed things up. I bought a gallon of milk and a gallon of orange juice. Cases of Coca-Cola, Sierra Mist Natural, Fanta Orange, and Lipton Iced Tea. On the student investigation sheet, the formulas for fructose, lactose, caffeine, sodium chloride and potassium chloride. Depending on what drink they chose, they may have a lot or a little work.
This lesson runs on our shortened day schedule, in a 42 minute period.
We are still working towards mastery of HS-PS1-7 with students eventually doing mass to mass conversions within a chemical reaction to prove the Law of Conservation of Mass. To get students there, they must know how to convert from mass to moles and then back. This lesson also utilizes Science and Engineering Practice 5, Using mathematical and computational thinking.
Opener -- Quiz
When students enter the class, I collect any Mole and Mass papers from the day before. I will spot check these and enter the grades in my gradebook while students are taking the quiz.
I ask students to get out a calculator and their periodic table. I provide extra blank periodic tables if students need them, along with loaner calculators. I generally don't mind students using their cell phone calculators, but I don't permit it on assessments due to the fantastic camera technology of the modern era.
When the class is ready, I explain that they are just finding the masses as indicated, and that when they are done they need to come to the front desk, turn in the quiz, and then pick up their Moles and Mass paper and today's paper.
I pass out the Molar Mass Quiz and let students get to work. Students work fairly quickly at the quiz. Not all label their units correctly as g/mol, but most do.
Students did very well on calculating the molar masses. Some students make an error on the Iron (III) nitrate and forget to distribute the 3 on the nitrate to the nitrogen as well as the oxygen. As seen in the example above, they do the opposite and distribute it to the nitrogen but not the oxygen.
When they turn in their papers, I show them to start figuring out the molar masses for the five chemicals at the top of the page.
When the last two or three students are finishing their quiz, I begin putting the pop cans out on the desk. This raises quizzical looks, but students don't ask questions until the the last quizzes are turned in.
Moles in Beverages
Once all students have finished the quiz, I explain the Stoichiometry of Soda procedure. Students will select a beverage, and then use the nutrition label to convert the number of grams of chemicals in the ingredients to moles.
We start with changing any measurements in milligrams to grams. Caffeine, sodium and potassium levels are generally in milligrams. If a student's beverage does not contain a particular ingredient, they X it out on their paper as seen here:
Students then finish the molar masses of all the chemicals, regardless of ingredients in their particular choice.
Students then use the number of grams from the label to convert into moles at the bottom of the page. No student needs to do all five conversions, as none of the drinks have all five chemicals. The Sierra Mist drinkers win the lottery, as they only have sugar and sodium as seen below.
Students do well with the conversions, occasionally looking back at their sample problems from the previous day, or asking for clarification. This is a hard activity to do whole class demonstrations as every label is different. As students are working, I'm circulating and making sure they are setting problems up correctly.
The crazy part was that in three of my classes, students didn't understand right away that they could have the drink in class. They were doing the chemistry, and then asked "Should we put the cans back up front?" When they realized the drinks were theirs, they started working a little harder, very appreciative of this little token and the class being a little more practical.
At the end of the period, I collected their papers to check their calculations. For the most part, they were completely flawless. Where they made errors, it was due to converting from milligrams to grams incorrectly.