Editing Molar mass (section)

== Precision and uncertainties ==
The precision to which a molar mass is known depends on the precision of the [[atomic mass]]es from which it was calculated (and very slightly on the value of the [[molar mass constant]], which depends on the measured value of the [[Dalton (unit)|dalton]]). Most atomic masses are known to a precision of at least one part in ten-thousand, often much better<ref name="AtWt"/> (the atomic mass of [[lithium]] is a notable, and serious,<ref>{{Greenwood&Earnshaw|page=21}}</ref> exception). This is adequate for almost all normal uses in chemistry: it is more precise than most [[chemical analysis|chemical analyses]], and exceeds the purity of most laboratory reagents.

The precision of atomic masses, and hence of molar masses, is limited by the knowledge of the [[Isotope|isotopic distribution]] of the element. If a more accurate value of the molar mass is required, it is necessary to determine the isotopic distribution of the sample in question, which may be different from the standard distribution used to calculate the standard atomic mass. The isotopic distributions of the different elements in a sample are not necessarily independent of one another: for example, a sample which has been [[Distillation|distilled]] will be [[Isotopic enrichment|enriched]] in the lighter [[isotope]]s of all the elements present. This complicates the calculation of the [[standard uncertainty]] in the molar mass.

A useful convention for normal laboratory work is to quote molar masses to two [[decimal place]]s for all calculations. This is more accurate than is usually required, but avoids [[rounding error]]s during calculations. When the molar mass is greater than 1000&nbsp;g/mol, it is rarely appropriate to use more than one decimal place. These conventions are followed in most tabulated values of molar masses.<ref>See, e.g., {{RubberBible53rd}}</ref><ref>
{{cite journal
 |title=Interpreting and propagating the uncertainty of the standard atomic weights (IUPAC Technical Report)
 |journal=Pure and Applied Chemistry
 |volume=90|issue=2|pages=395–424
 |date=2018-01-04
 |doi=10.1515/pac-2016-0402
 |first1=Antonio |last1=Possolo
 |first2=Adriaan M. H. |last2=van der Veen
 |first3=Juris |last3=Meija
 |first4=D. Brynn |last4=Hibbert
 |s2cid=145931362
 |url=https://nrc-publications.canada.ca/eng/view/fulltext/?id=f1f2ca7f-9b5f-4da0-bbba-d196cc190911
 |doi-access=free
}}</ref>