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Relating Mass to Numbers of Atoms The mole, Avogadro’s number, and molar mass provide the basis for relating masses in grams to moles.

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The Mole The mole is the SI unit for amount of a substance. A mole (mol) is the amount of a substance that contains as many particles as there are atoms in exactly 12 g of carbon-12. The mole is a counting unit, just like a dozen eggs is.

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Just remember to think of the “mole” as a counting unit. We don’t usually order 12 or 24 doughnuts; we order one dozen or two dozen. Similarly, a chemist may want 1 mol of carbon, or 2 mol of carbon, or 2.567 mol of carbon. In the following slides, you will see how the mole relates to masses of atoms and compounds.

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Avogadro’s Number The number of particles in a mole has been experimentally determined to have a value of 6.02 x 10 23. This means that exactly 12 g of carbon-12 contains 6.02 x 10 23 carbon-12 atoms.

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Avogadro’s number (6.02 x 10 23 ) is the number of particles in exactly one mole of pure substance. Amadeo Avogadro was an Italian scientist noted to be one of the founders of physical chemistry. He was actually a physics professor but he experimented in both physics and chemistry using mathematics to base most of his findings. He is most famous for his hypothesis known as Avogadro’s Law. His law simply states that at a fixed temperature and pressure, equal volumes of gases contain the same number of molecules. The number 6.02214199 x 10 23 is called Avogadro’s number, in honor of Amadeo Avogadro, who was the first person to argue in favor of the existence of atoms.

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Molar Mass The mass of one mole of a pure substance is called the molar mass. It is usually written in the unit g/mol. The molar mass of an element is equal to the atomic mass of the element in atomic mass units (amu). (molar mass of Li is 6.94 g/mol).

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Chemists use molar mass as a conversion factor in chemical calculations. For example, the molar mass of helium is 4.00 g He/1 mol He. To find out how many grams of helium are in two moles of helium, multiply by the molar mass.

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