Molecular Weight Definition

Molecular Weight and How to Calculate It

Molecular weight is the total mass of a single molecule.
Molecular weight is the total mass of a single molecule. BlackJack3D, Getty Images

Molecular Weight Definition

Molecular weight is a measure of the sum of the atomic weight values of the atoms in a molecule. Molecular weight is used in chemistry to determine stoichiometry in chemical reactions and equations. Molecular weight is commonly abbreviated by M.W. or MW. Molecular weight is either unitless or expressed in terms of atomic mass units (amu) or Daltons (Da).

Both atomic weight and molecular weight are defined relative to the mass of the isotope carbon-12, which is assigned a value of 12 amu.

The reason the atomic weight of carbon is not precisely 12 is because it is a mixture of isotopes of carbon.

Sample Molecular Weight Calculation

The calculation for molecular weight is based on the molecular formula of a compound (i.e., not the simplest formula, which only includes the ratio of types of atoms and not the number). The number of each type of atom is multiplied by its atomic weight and then added to the weights of the other atoms.

For example, the molecular formula of hexane is C6H14. The subscripts indicate the number of each type of atom, so there are 6 carbon atoms and 14 hydrogen atoms in each hexane molecule. The atomic weight of carbon and hydrogen may be found on a periodic table.

atomic weight of carbon: 12.01

atomic weight of hydrogen: 1.01

molecular weight = (number of carbon atoms)(C atomic weight) + (number of H atoms)(H atomic weight)

molecular weight = (6 x 12.01) + (14 x 1.01)

molecular weight of hexane = 72.06 + 14.14

molecular weight of hexane = 86.20 amu

How Molecular Weight Is Determined

Empirical data on the molecular weight of a compound depends on the size of the molecule in question. Mass spectrometry is commonly used to find the molecular mass of small to medium-sized molecules.

The weight of larger molecules and macromolecules (e.g., DNA, proteins) is found using light scattering and viscosity. Specifically, the Zimm method of light scattering and the hydrodynamic methods dynamic light scattering (DLS), size-exclusion chromatography (SEC), diffusion ordered nuclear magnetic resonance spectroscopy (DOSY), and viscometry may be used.

Molecular Weight and Isotopes

Note, if you are working with specific isotopes of an atom, you should use the atomic weight of that isotope rather than the weighted average provided from the periodic table. For example, if instead of hydrogen, you are dealing only with the isotope deuterium, you use 2.00 rather than 1.01 for the atomic mass of the element. Ordinarily, the difference between the atomic weight of an element and the atomic weight of one specific isotope is relatively minor, but it can be important in certain calculations!

Molecular Weight Versus Molecular Mass

Molecular weight is often used interchangeably with molecular mass in chemistry, although technically there is a difference between the two. Molecular mass is a measure of mass and molecular weight is a measure of force acting on the molecular mass. A more correct term for both molecular weight and molecular mass, as they are used in chemistry, would be "relative molecular mass".