KentChemistry HOME


Custom Search

More Gas Law links


Molar Mass

(Molar Mass, Molecular Weight, Gram Formula Mass)


Mole Conversions



Molar Mass is the mass of one mole of a substance (6.02 x 1023 formula units). The unit for molar mass (note it is the mass of a mole) is grams/mole.

Atomic Weight is the molar mass of an element.

Formula Weight is the molar mass of an IONIC compound.

Molecular Weight is  the molar mass of a COVALENT compound.

Molecular Mass is the mass of a given molecule (NOT MOLES of molecules). The units are atomic mass units (amu).

1amu =1.660 538 78210–27 kg                1 amu is 1/12 the mass of a carbon-12 atom which has a mass of 12.0000.


Reading Simple Chemical Formulas- Each element begins with a CAPITAL letter. The subscript belongs to the element it follows and indicates how many of that atom there are. If there is only 1 atom we do not use subscripts.

H2O      2 hydrogen    1 oxygen

Reading Compounds that have (parenthesis's).

When there is parenthesis's around a group of atoms we must multiply each atom by the subscript to get the total number of each element.

Ca(NO3)2     1 Calcium  2 Nitrogen and 6 Oxygen -->  3 x 2


How do we determine molar mass?

The molar mass is calculated by taking the sum of the atomic weights of all the atoms which form the molecule.

First determine the number of each atom in the formula.

1. Water is made up of 2 hydrogens and 1 oxygen.

Find the molar mass.

2. Multiply the number of each atom by its atomic weight (found on the Periodic Table of the Elements). 


H     2  x 1.00794   =2.01588 Add them up  Molecular Weight=18.01528g/mol  

For our class we can round to 1 decimal place so 18.0g/mol

O     1  x 15.9994 = 15.9994


C     6    x  12.0107=  72.0642 Add them up  Molecular Weight=180.15588g/mol  

rounded to 180.2g/mol

H     12  x  1.00794=  12.09528
O     6    x  15.9994=  95.9964


Ca    1   x   40.078   =  40.078 Add them up  Molecular Weight=164.0878g/mol  

rounds to 164.1g/mol

N     2   x   14.0067 =  28.0134
O     6   x  15.9994  =  95.9964


Highlight box to reveal the molar masses
Formulas Highlight to Reveal Molar Masses (g/mol)
CO2 44
HNO3 63
CCl4 182
Na2SO4 142
C12H22O11 342
NH4OH 35
Cu(NO3)2 125
MgBr2 184
Be(OH)2 43
LiBr 87
CaBr2 200
KNO3 101
KF 58
Na2CO3 106
Ca(OH)2 74.1
CuSO4 159.6
C3H4O3 88.0

Mole Conversions

Chemical Demonstration Videos