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.
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 = 106.0 g/mole mass
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 = 164.1 g/mol mass
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
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 = 159.7 g/mol mass
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 169.9 = mass 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 639.2 = mass 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 239.1 = mass 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) 102.9 g/mol 2) 303.3 g/mol 3) 74.1 g/mol 4) 164.0 g/mol 5) 96.0 g/mol 6) 180.0 g/mol 7) 357.4 g/mol 8) 68.1 g/mol 9) 183.4 g/mol 10) 126.9 g/mol