Moles and Molar Mass
The mole is the "counting unit" used by chemists to indicate the number of atoms, ions,
molecules, or formula units present in a particular chemical sample. The mole is similar to
other counting units that you've used before....pair (2), dozen (12), and gross (144). One
mole of a compound contains Avogadro's number (6.022 x 1023) of molecules (molecular
compound) or formula units (ionic compound). The molar mass of a compound tells you the mass
of 1 mole of that substance. In other words, it tells you the number of grams per mole of a
compound. The units for molar mass are, therefore, grams/mole.
To find the molar mass of a compound:
1.
2.
3.
Use the chemical formula to determine the number of each type of atom present in the
compound.
Multiply the atomic weight (from the periodic table) of each element by the number of
atoms of that element present in the compound.
Add it all together and put units of grams/mole after the number.
Example: Find the molar mass of sodium carbonate, Na2CO3.
Na
2
x
23.0
= 46.0
C
1
x
12.0
= 12.0
O
3
x
16.0
= 48.0
molar
mass
= 106.0 g/mole
For many (but not all) problems, you can simply round the atomic weights and the molar mass
to the nearest 0.1 g/mole. HOWEVER, make sure that you use at least as many significant
figures in your molar mass as the measurement with the fewest significant figures. In other
words, never let your molar mass be the measured value that determines how many signficant
figures to use in your answer!
Example: What is the molar mass of calcium nitrate, Ca(NO3)2?
Ca
1
x
40.1
= 40.1
N
2
x
14.0
= 28.0
O
6
x
16.0
= 96.0
molar
mass
= 164.1 g/mol
Notice that you must be very careful when you're counting the number of atoms present in
this compound. The subscript "2" after the parentheses indicates that there are 2 nitrate
ions (NO3-). In order to figure out how many N atoms are present, you must multiply the
subscripts (in this case 1 x 2 = 2). In order to figure out how many O atoms are present, you
must multiply the subscripts (in this case 3 x 2 = 6). Notice that the subscript outside the ( )
affects only the atoms inside the ( ) and not the Ca ion.
Practice Problems
Calculate the molar mass for each of the following compounds:
1.
2.
3.
4.
Fe2O3
AgNO3
Pb(Cr2O7)2
Ca(ClO4)2
1. The molar mass of Fe2O3 is 159.7 g/mol.
Fe
2
x
55.85
=
111.70
O
3
x
16.0
=
+ 48.0
molar
mass
= 159.7 g/mol
2. The molar mass of AgNO3 is 169.9 g/mol.
Ag
1
x
107.9
=
107.9
N
1
x
14.0
=
14.0
O
3
x
16.0
=
+ 48.0
molar
mass
=
169.9
g/mol
3. The molar mass of Pb(Cr2O7)2 is 639.2 g/mol.
Pb
1
x
207.2
=
207.2
Cr
4
x
52.0
=
208.0
O
14
x
16.0
=
+ 224.0
molar
mass
=
639.2
g/mol
4. The molar mass of Ca(ClO4)2 is 239.1 g/mol.
Ca
1
x
40.1
=
40.1
Cl
2
x
35.5
=
71.0
O
8
x
16.0
=
+ 128.0
molar
mass
=
239.1
g/mol
Molar Mass Practice Worksheet
Find the molar masses of the following compounds:
1)
NaBr
2)
PbSO4
3)
Ca(OH)2
4)
Na3PO4
5)
(NH4)2CO3
6)
C6H12O6
7)
Fe3(PO4)2
8)
(NH4)2S
9)
Zn(C2H3O2)2
10)
AgF
Solutions to the Molar Mass Practice Worksheet:
Important note to students: All of the units given here are “grams per mole”, which may
be abbreviated as “g/mol”, “grams/mol”, or “g . mol-1”, depending on how your teacher
likes to see it written. They all mean the same thing, but it’s probably a good idea to
use whatever your teacher showed you in class. Also, remember that if you don’t use
units in your answer, the answer is wrong!
All answers are rounded to the nearest 0.1 grams.
1)
2)
3)
4)
5)
6)
7)
8)
9)
10)
102.9 g/mol
303.3 g/mol
74.1 g/mol
164.0 g/mol
96.0 g/mol
180.0 g/mol
357.4 g/mol
68.1 g/mol
183.4 g/mol
126.9 g/mol