Editing Molecular mass (section)

== Calculation ==
Molecular masses are calculated from the [[atomic mass]]es of each [[nuclide]] present in the molecule, while [[Molar mass|molar masses]] and relative molecular masses (molecular weights) are calculated from the [[standard atomic weight]]s<ref>{{cite web | title = Atomic Weights and Isotopic Compositions for All Elements | url = http://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=&all=all&ascii=html&isotype=some | publisher = [[NIST]] | access-date = 2007-10-14}}</ref> of each [[Chemical element|element]]. The standard atomic weight takes into account the [[Isotope|isotopic distribution]] of the element in a given sample (usually assumed to be "normal"). For example, [[water (molecule)|water]] has a molar mass of 18.0153(3) g/mol, but individual water molecules have molecular masses which range between 18.010&nbsp;564&nbsp;6863(15)&nbsp;Da (<sup>1</sup>H{{su|b=2}}<sup>16</sup>O) and 22.027&nbsp;7364(9)&nbsp;Da (<sup>2</sup>H{{su|b=2}}<sup>18</sup>O).

Atomic and molecular masses are usually reported in [[dalton (unit)|dalton]]s, which is defined in terms of the mass of the [[isotope]] <sup>12</sup>C (carbon-12). However, the name ''[[Dalton (unit)|unified atomic mass unit]]'' (u) is still used in common practice. Relative atomic and molecular masses as defined are [[dimensionless]]. Molar masses when expressed in [[Gram|g]]/[[Mole (unit)|mol]] have almost identical numerical values as relative atomic and molecular masses. For example, the molar mass and molecular mass of [[methane]], whose molecular formula is CH<sub>4</sub>, are calculated respectively as follows:

{| class="wikitable plainrowheaders"
!colspan=4|Molar mass of CH<sub>4</sub>
|-
!  !! Standard atomic weight       !! Number<br>of atoms !! Total molar mass (g/mol) <br> or molecular weight (unitless)
|-
| C || {{Atomic mass|C}} || 1      || style="text-align:right;"|{{Atomic mass|C}}
|-
| H || {{Atomic mass|H}} || 4      || style="text-align:right;"|{{Molecular mass|H|4}}
|-
| CH<sub>4</sub> ||      ||        || style="text-align:right;"|{{Molecular mass|C|1|H|4}}
|-
!colspan=4|Molecular mass of <sup>12</sup>C<sup>1</sup>H<sub>4</sub>
|-
!  !! Nuclide mass (Da or u)      !! Number<br>of atoms !! Total molecular mass (Da or u)
|-
| <sup>12</sup>C || 12.0000 || 1      || style="text-align:right;"|12.0000
|-
| <sup>1</sup>H || 1.007825 || 4      ||style="text-align:right;"| 4.0313
|-
| CH<sub>4</sub> ||      ||        ||style="text-align:right;"| 16.0313
|}

The uncertainty in molecular mass reflects variance (error) in measurement not the natural variance in isotopic abundances across the globe. In high-resolution [[mass spectrometry]] the mass isotopomers <sup>12</sup>C<sup>1</sup>H<sub>4</sub> and <sup>13</sup>C<sup>1</sup>H<sub>4</sub> are observed as distinct molecules, with molecular masses of approximately 16.031 Da and 17.035 Da, respectively. The intensity of the mass-spectrometry peaks is proportional to the isotopic abundances in the molecular species. <sup>12</sup>C <sup>2</sup>H <sup>1</sup>H<sub>3</sub> can also be observed with molecular mass of 17 Da.