The mass of a molecule is measured in terms of its “molecular weight.” The molecular weight of a molecule is calculated by adding up the weights of each atom that makes it up.
For instance, water, with two hydrogen atoms and one oxygen atom, it has a molecular weight of 18 (i.e., 2 + 16).
The mass of a substance or molecule is often measured with a mass spectrometry device.
Molecular weight is also known as formula mass, relative molecular mass, or molecular mass.
For ease, important points are listed in the table below:
| Definition | The molecular weight of a substance is the weight in atomic mass units of each atom in a given formula. |
| Units | either unitless or expressed in terms of atomic mass units (AMU) or Daltons (Da). One AMU = one g/mol |
| Abbreviation | MW |
Example of Molecular Weight
Acetylene (C2H2) is a chemical compound composed of two carbon and two hydrogen atoms. It has a molecular weight of 26.04 g/mol.
Mass of carbon = 12.01 g/mol
The mass of hydrogen is 1.00794 g/mol.
Molecular weight of acetylene = 2 x (12.011) + 2 x (1.00784) = 26.04 g/mol