Teacher's Notes

Note #1 A Dozen Moles

Students seem to have difficulty grasping the concept of the definition of a mole. A possible solution is to compare the concept of a dozen to the concept of a mole.

Everyone knows that there are 12 particles (eggs, pencils, people, etc.) in a dozen. The students need to be able to transfer this to the concept of 6.022 x 10²³ particles (atoms, molecules, etc.) in a mole. This can be encouraged by bringing in a dozen eggs, a dozen pencils, and get the students into groups of a dozen. These dozens all contain 12 particles, but they all look different. You should also bring a mole of salt, sugar, or whatever else you have around. To get a mole, just find the molecular weight of the compound and weigh that much out.

By looking at these different dozens and moles students will understand that the mole is just a word used to describe how many particles you have. They will also understand that each mole of each different substance can look different, just like a dozen people look different than a dozen eggs.

Note#2 Atoms vs. Molecules

Students seem to have difficulty converting from a mole of a compound into atoms and molecules.

For example, H₂0

If you have 1 mole of water, you have 6.022x10²³ water molecules, but you have twice as many hydrogen atoms. This seems to cause difficulty to some students. An analogy back to the dozen may help.

This topic could possibly be discussed using the dozen people comparison. If you have a dozen people, there are 12 people. People are composed of 2 legs and 2 arms and 1 head. Therefore, there are 24 legs, 24 arms, and 12 heads. Water is composed of 2 Hydrogens and 1 Oxygen, so 1 mole of water is 6.022x10²³ molecules of water, twice that for Hydrogen atoms, and 6.022x10²³ atoms of Oxygen

Note #3 Units

Many students tend to leave units out when they are doing calculations. Students must write units down in order to understand what they are doing and to make sure the answer is correct. Many students will just use 6.022x10²³ as a number, and this may cause them to miss many questions because they are not sure what they are doing. If they are asked to find atoms of Hydrogen in one mole of water, but they simply multiply by Avogadro's number, the answer will be incorrect.

Please, make sure students always write the units down for every problem. This makes it easier on the students and the teacher. When students write down their units, it is easier to see where they made a mistake. Chemistry is very simple when units are written down. As long as you cancel out the units you don't need and keep the ones you do need, you will get the correct answer.

Note#4 Practice

The best way for a student to learn the concept of the mole is just practice. Students should do problems converting grams to moles, moles to molecules, and molecules to atoms. This can also be done the reverse. By practicing this concept, students will be able to move on to the topic of stoichiometry with less difficulty.