Molar Mass
Chemistry 11
Ms. McGrath
Molar Mass
Molar mass (M) is the mass of one mole
(6.02 x 1023 particles) of a substance
numerically equal to the element’s
average atomic mass (u). It is
expressed in g / mol.
Molar Mass
Molar masses of common chemical
compounds that you might find in the
chemistry laboratory can range between
18 grams/mole for compounds like
water to hundreds of grams per mole
for more complex chemical compounds.
Molar Mass
The lightest possible chemical that one can
have under normal conditions is hydrogen
gas, or H2. There is no limit to how heavy a
chemical compound can be - it is not
uncommon for macromolecules (large
organic or bioorganic compounds such as
DNA) to weigh thousands of grams per
mole.
Molar Mass
How do I find Molar Mass?
The molar mass of elements is found by
looking at the atomic mass of the element
on the periodic table.
For example, if you want to find the molar
mass of carbon, you would find the atomic
mass of carbon on the periodic table, and this
is equal to the molar mass in grams per mole.
So, carbon has a molar mass of 12.01 grams
per mole.
Molar Mass
How do I find Molar Mass?
There are a few exceptions to this rule.
 In the case of H, N, O, F, Cl, Br, and I, the
element is diatomic, meaning that each
molecule of the element has two atoms of that
element stuck together.
 P is normally found in clumps of four atoms,
and S is found in clumps of eight atoms.
Still, aside from the exceptions above, all
elements have the same molar mass as the
atomic masses on the periodic table.
Molar Mass
How do I find Molar Mass?
Molar masses of chemical compounds
are equal to the sums of the molar
masses of all the atoms in one molecule
of that compound.
ex: BeO
MBeO = 9.01 g/mol + 16.00 g/mol
= 25.01 g/mol
Molar Mass
How do I find Molar Mass?
ex: Ca3(PO4)2
MCa x 3 = (40.08 g/mol) x 3
= 120.24 g/mol
MP x 2 = (30.97 g/mol) x 2
= 61.94 g/mol
MO x 8 = (16.00 g/mol) x 8
= 128.00 g/mol
MCa3(PO4)2 = 120.24 g/mol + 61.94 g/mol + 128.00 g/mol
= 310.18 g/mol
Molar Mass
Practice - Molar Mass
Page 57 #16, 17, 18, 19
Molar Mass
Converting from Moles to Mass
Mass = Number of moles x Molar Mass
m=nxM
Molar Mass
Converting from Moles to Mass
A flask contains 0.750 mol of carbon dioxide gas,
CO2. What mass of carbon dioxide gas is in this
sample?
m=nxM
MCO2 = 2 x (16.00 g/mol) + 12.01 g/mol
= 44.01 g / mol
m=nxM
= (0.750 mol) x (44.01 g/mol)
= 33.0 g
Molar Mass
Practice - Converting from Moles to Mass
Page 59 #20, 21, 22, 23
Molar Mass
Converting from Mass to Moles
Mass = Number of moles x Molar Mass
m=nxM