Molecular mass

Molecular mass or molecular weight is the mass of a molecule. It is calculated as the sum of the mass of each constituent atom multiplied by the number of times that atom occurs in the molecule. The molecular mass of small to medium size molecules, measured by mass spectrometry, determines stoichiometry. For large molecules such as proteins, methods other than mass spectrometry may be employed.

Calculation

Molecular mass is calculated using the molecular formula, which gives the number of each type of atom in a molecule. The atomic mass of each atom is looked up on the periodic table and then multiplied by the number of that type of atom in the molecule. The atomic masses of all the atoms are then added together to give the molecular mass.

For example, the molecular mass of water, which has the molecular formula H2O, is calculated like this:

(2 x atomic mass of hydrogen) + (1 x atomic mass of oxygen) = molecular mass of water

Applications

Molecular mass is used in stoichiometry (the calculation of quantities in chemical reactions) in the same way that atomic mass is used in the calculation of the mass of a single atom. It is also used in the calculation of the molar mass of a compound.

See also

• Atomic mass
• Molar mass
• Mass spectrometry