Stoichiometry Tutorial
Finding Molar Mass

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. Use the chemical formula to determine the number of each type of atom present in the compound.
  2. Multiply the atomic weight (from the periodic table) of each element by the number of atoms of that element present in the compound.
  3. Add it all together and put units of grams/mole after the number.

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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. Fe2O3
  2. AgNO3
  3. Pb(Cr2O7)2
  4. Ca(ClO4)2

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Answers

Tutorial on Using Molar Mass to Convert between Grams and Moles