This lab came from Ian Guch’s 24 Lessons That Rocked the World, Chapter 6 – Moles, Molecules, and Grams Lab. Before class I prepared film canisters with 5 grams of salt, sugar, water, gypsum sand and isopropyl alcohol. The students had to determine how many moles and molecules of each substance were in the each container. We used a digital kitchen scale and a triple beam balance (pictured above) to measure the mass of each sample which was poured onto a watchglass. The kitchen scale is not terribly percise, only measuring to the nearest gram, but this lab was more about how to do the calculations then making precise measurements. They then had to find the molar mass, which involved looking up the atomic mass of each atom in the chemical formula for each sample and adding them up. Dividing the 5 grams (or whatever the mass was that they measured) by the molar mass told them how many moles they had, and then all that’s left is to multiply by Avogadro’s number (6.022×1023) which is the number of molescules in one mole of anything, to get the number of molecules. This lab mimicked the homework they had for chapter 3 from Modern Chemistry, but put a real substance in front of them. The older kids, mostly juniors, had no problem with this lab but as I suspected, the younger students weren’t sure how to do the calculations, specifically how to enter avogadro’s number into their calculator so this was a good activity to do in class.
This activity showed that 5 grams of water has a lot more molecules in it then 5 grams of sugar since each sugar molecule (model on left) has a larger mass, it therefore takes fewer of the, then say water molecules, to get to 5 grams. Students also learned how to use their calculators.
There is a great video by Tyler Dewitt explaining how to do these calculations.