Here are two very common questions about moles. First: we'll learn how to calculate the mass of a single atoms, answering the question "What is the mass in grams of a single atom of oxygen?" Then, we'll use moles to calculate a mass relationship between two elements, answering the question, "What mass of Mercury in grams has the same number of atoms at 64.2 grams of Calcium?" These two types of mole problems are very common on homework, in textbooks, on quizzes, and particularly on tests and exams.