Relating Mass to Numbers of Atoms
•The mole, Avogadro’s number, and molar mass
provide the basis for relating masses in grams to
moles.
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.
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.
Avogadro’s Number
•The number of particles
in a mole has been
experimentally
determined to have a
value of 6.02 x 1023. This
means that exactly 12 g
of carbon-12 contains
6.02 x 1023 carbon-12
atoms.
•Avogadro’s number (6.02 x 1023) 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 1023 is called Avogadro’s number, in honor
of Amadeo Avogadro, who was the first person to argue in favor of the
existence of atoms.
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).
•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.
4.00 g He
2.00 mol He x
 8.00 g He
1 mol He