= Molar mass
{title2=$M$}
{wiki}
<Molar mass> turns a weighed sample into a count of specified particles. For a pure <chemical substance> with specified <chemical formula> and <isotopic abundances>, its <molar mass> is
$$
M=\frac{m}{n},\qquad n=\frac{m}{M},
$$
where $m$ is the sample <mass> and $n$ its <amount of substance>. The <SI> unit is $\mathrm{kg\,mol^{-1}}$; the common laboratory unit is $\mathrm{g\,mol^{-1}}$, with $1\,\mathrm{g\,mol^{-1}}=10^{-3}\,\mathrm{kg\,mol^{-1}}$. Equal values of <amount of substance> correspond to equal counts of specified entities, but heavier entities give greater <masses>.
Combining this definition with <molecular count and amount of substance> gives
$$
N=\frac{mN_{\mathrm A}}{M},\qquad \bar m_{\rm particle}=\frac{m}{N}=\frac{M}{N_{\mathrm A}},\qquad N_{\mathrm A}=6.02214076\times10^{23}\,\mathrm{mol^{-1}}.
$$
The <Avogadro constant> is exact under the modern definition of the <mole>. The <mass> $M/N_{\mathrm A}$ is the mean <mass> per specified entity; if <isotopes> differ within the sample, individual <molecules> need not all have that mean <mass>.<The International System of Units (SI), ninth edition, SI Brochure>[§2.3.1, The mole, printed p. 131: exact Avogadro constant, specified entities, and M(X) = N_A m(X)]{ref}
Specify what $n$ counts: <water> <molecules>, <hydrogen> <chemical atoms>, and <oxygen> <chemical atoms> give different values of <amount of substance> for the same sample. For a <mixture>, dividing total <mass> by total <amount of substance> of its chosen entities gives a composition-dependent mean. A <polymer> sample can contain many chain <molar masses>; its <number-average molar mass> and <weight-average molar mass> answer different averaging questions. Relating either to <degree of polymerization> also requires a unit-mass convention and end-group accounting.
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