Sal introduces chemistry students to the concept of "moles." He points out that a mole is a unit of measurement that helps a student to convert between grams and atomic mass units of any substance. Sal uses the element Aluminum to illustrate this point. The mass number for Aluminum is 26.98, so one mole of Aluminum is equal to 26.98 grams. As with many of his previous videos, Sal uses the Periodic Table to help explain these concepts.