Editing Molar mass (section)

== Molar masses of compounds ==
The molar mass of a [[Chemical compound|compound]] is given by the sum of the [[relative atomic mass]] {{math|''A''{{sub|r}}}} of the [[atom]]s which form the compound multiplied by the [[molar mass constant]] <math>M_u \approx 1 \text{ g/mol}</math>:
: <math>M = M_{\rm u} M_{\rm r} = M_{\rm u} \sum_i {A_{\rm r}}_i.</math>

Here, {{math|''M''{{sub|r}}}} is the relative molar mass, also called formula weight. For normal samples from earth with typical isotope composition, the [[standard atomic weight]] or the conventional atomic weight can be used as an approximation of the relative atomic mass of the sample. Examples are: <math chem display=block>\begin{array}{ll}
M(\ce{NaCl}) &= \bigl[22.98976928(2) + 35.453(2)\bigr] \times 1 \text{ g/mol} \\
             &= 58.443(2) \text{ g/mol} \\[4pt]
M(\ce{C12H22O11})  &= \bigl[12 \times 12.0107(8) + 22 \times 1.00794(7) + 11 \times 15.9994(3)\bigr] \times 1 \text{ g/mol} \\ 
                   &= 342.297(14) \text{ g/mol}
\end{array}</math>

An average molar mass may be defined for mixtures of compounds.<ref name="GreenBook" /> This is particularly important in [[polymer science]], where there is usually a [[molar mass distribution]] of non-uniform polymers so that different polymer molecules contain different numbers of [[monomer]] units.<ref>{{cite journal | title = International union of pure and applied chemistry, commission on macromolecular nomenclature, note on the terminology for molar masses in polymer science| year = 1984 | journal = Journal of Polymer Science: Polymer Letters Edition| volume = 22 | pages = 57 | issue=1 | doi=10.1002/pol.1984.130220116 |bibcode = 1984JPoSL..22...57.}}</ref><ref>{{cite book | last = Metanomski | first =  W. V. | title = Compendium of Macromolecular Nomenclature | year = 1991 | publisher = [[Blackwell Science]] | location = Oxford | pages = 47–73 | isbn = 0-632-02847-5}}</ref>