Definition:Molecular Weight

Definition

Let $S$ be a substance.

Let $S$ be homogeneous.

The molecular weight of $S$ is defined as:

$W_S = m_S \times \dfrac {12} {m_C}$

where:

$m_S$ is the (arithmetic) mean mass of one molecule of $S$

$m_C$ is the mass of one molecule of carbon-12.

It is understood that the atoms that compose the molecules of $S$ may be of different isotopes according to their relative natural concentrations at the source of wherever $S$ came from.

Hence the use of the (arithmetic) mean mass in the definition of $m_S$.

Linguistic Note

The term molecular weight is a misnomer, as what is being defined here is a mass.