\begin{definition}[Definition:Molecular Weight]
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 $m_S$.
\end{definition}