= Relative molecular mass
{title2=$M_{\rm r}$}
The <relative molecular mass> is the <dimensionless> ratio of a specified mean <molecule> <mass> to the <unified atomic mass unit>, written $m_{\rm u}$ when treated as a <physical quantity>:
$$
M_{\rm r}=\frac{\bar m_{\rm molecule}}{m_{\rm u}},\qquad M=M_{\rm r}M_{\rm u}.
$$
Thus a <relative molecular mass> of approximately $18.015$ and a <molar mass> of approximately $18.015\,\mathrm{g\,mol^{-1}}$ describe related quantities with different units. The conversion uses the <molar mass constant>, rather than identifying a <dimensionless> number with a <mass> per <mole>.
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