How to calculate molecular mass?
- Determine the formula of the compound
- Use the periodic table, determine the atomic mass of each element present in the formula
- Multiply the atomic mass of each element by the number of atoms
- Finally, add the atomic masses
Example: How to find the molecular mass of water?
- First, determine the formula for water.
- The formula for water is H2O.
- Use the periodic table to look up the atomic mass for the elements in the formula.
- You must then count how many atoms of each type are present using the formula.
|hydrogen||1.008 amu||2||2(1.008) = 2.016 amu|
|oxygen||16.00 amu||1||1(16.00) = 16.00 amu|
|Total the column
= 18.02 amu (rounded)
Molecular mass can also be expressed in terms of moles (g/mol)
|Substance||Formula||Molecular Mass||Molar Mass|
|Water||H2O||18.02 amu||18.02 g/mol|
|Carbon dioxide||CO2||44.01 amu||44.01 g/mol|
|Oxygen||O2||32.00 amu||32.00 g/mol|
Amu and g/mol are equivalent to each other.
The difference between molecular mass and molar mass