"How many moles of calcium atoms does it contain? The answer is one half of a mole, because 1 mole is equal to 6.02 x 1023, so a half of a mole would be equal to 3.01 x 1023 atoms. Okay, so if it contains 0.5 moles of calcium atoms, how much mass would that have? The answer is about 20 grams."
