Modern Chemistry Chapter 7, Chemical Formulas and Chemical Compounds was fairly boring, the first half of the chapter was all about chemical names which really put me to sleep. I’m not about to memorize that so I’m not asking my students to either. The last part of the chapter was about molar mass and percent composition, some of which we’d already covered earlier this semester. I did give the students a worksheet about naming covalent compounds that I got from Covalent Bonding Interactive Graphic Organizers for the Chemistry Notebook or Lapbook by Bond with James (teacherspayteachers.com). The worksheet had half a page explaining naming rules and then half a page where the students had to name the compound or write out the formula. One of the students did find one typo on that page, the answer key had a different compound than the question sheet.
The lab for this class, Chapter 7 Percent Composition Lab, was taken from Ian Guch’s 24 Lessons that Rocked the World. I really liked this lab, it was fairly straight forward, measure the mass of your container, the mass of container + epson salts, heat epson salts then find mass of what’s left. Heating the epson salts drives out the water and the change in mass is very significant, so its the not typical small change that you might miss if you’re not careful. The visible change is also very big and got lots of suprised ‘Wow!’s from the students. I hadn’t done this lab before so I hadn’t known what to expect either. One of the students wondered out loud what it would like under the microscope, so I pulled it out and we took some pictures. The photo on the left with the large crystals is hydrated magnesium sulfate (epsom salts before heating), and the photo on the right is anhydrous magnesium sulfate (epsom salts after heating). The microscope was on the same magnification for both images so you can see the change was pretty drastic.
The lab handout steps the students through the process of calculating the percent composition of water for the epsom salts and then finding the chemical formula. The last page of the lab includes a worksheet with additional practice in calculating percent composition which I assigned as homework.
This is a great lab for homeschoolers since all you need is epson salts, which you can buy at a grocery store, a scale (and a kitchen scale would probably work since the mass change is so large) and a heat source. We didn’t get exactly the right numerical answer for the lab, our numbers were all a little low but the students quickly realized that meant they probably didn’t heat the salts long enough so there was still some water left in the cruicible. We left the lids on the cruicibles while heating, so we didn’t watch the reaction and just heated the 10 minutes stated in the lab handout. I would like to try this again with the lids off so we could tell when the reaction is done and see if we get better numbers. The only safety concern with this lab is the heat source.
Tyler DeWitt has a nice set of videos on how to calculate percent composition.