Molecular mass 19836 225630137 2008-07-14T17:14:45Z Yamakiri 5245118 revise {{Otheruses4|the mass of a single molecule|the mass of a mole of a substance commonly used in basic stoichiometric calculations|Molar mass}} [[Image:Water molecule.svg|thumb|right|Assuming Hydrogen and oxygen are standard weights in this image (as opposed to deuterium oxide) the '''molecular mass''' should be 18.01528&nbsp;u.]] The '''molecular mass''' (abbreviated '''Mr''') of a [[chemical compound|substance]], more commonly referred to as '''molecular weight''' and abbreviated as '''MW''', is the [[mass]] of one [[molecule]] of that substance, relative to the [[unified atomic mass unit]] u<ref>http://www.fordhamprep.org/gcurran/sho/sho/lessons/lesson56.htm</ref> (equal to 1/12 the mass of one [[atom]] of [[carbon-12]]<ref>http://www.webqc.org/mmcalc.php</ref>). This is distinct from the '''relative molecular mass''' of a molecule, which is the ratio of the mass of that molecule to 1/12 of the mass of carbon 12 and is a dimensionless number. Relative molecular mass is abbreviated to '''Mr'''. Molecular mass differs from more conventional measurements by taking into account different isotopic compositions, and as a result Molecular mass is generally more accurate then molar mass, but less used. == Definition == There are varying interpretations of this definition. Many chemists use molecular mass as a synonym of [[molar mass]]<ref>http://www.mrcoulter.com/REG_LECTURES/09formula.pdf</ref>, differing only in units (see average molecular mass below). A stricter interpretation does not [[equate]] the two, as the mass of a single molecule is not the same as the average of an [[ensemble]]. Because a mole of molecules may contain a variety of molecular masses due to natural [[isotope]]s, the average mass is usually not identical to the mass of any single molecule. The actual numerical difference can be very small when considering small molecules and the molecular mass of the most common [[isotopomer]] in which case the error only matters to [[physicists]] and a small subset of highly specialized [[chemists]]; however it is always more correct, accurate and consistent to use [[molar mass]] in any bulk [[stoichiometric]] calculations. The size of this [[margin of error|error]] becomes much larger when considering larger molecules or less [[abundance|abundant]] isotopomers. The molecular mass of a molecule which happens to contain heavier isotopes than the average molecule in the sample can differ from the molar mass by several mass units. == Average mass == The ''average molecular mass'' (sometimes abbreviated as ''average mass'') is another [[variation]] on the use of the term molecular mass. The average molecular mass is the abundance weighted [[mean]] (average) of the molecular masses in a [[sample]]. This is often closer to what is meant when "molecular mass" and "molar mass" are used [[synonym|synonymously]] and may have [[derived]] from shortening of this term. The average molecular mass and the molar mass ''of a particular substance in a particular sample'' are in fact [[numerically]] [[identical]] and may be [[interconverted]] by [[Avogadro's constant]]. It should be noted, however, that the molar mass is almost always a [[computed figure]] derived from the [[standard atomic weight]]s, whereas the average molecular mass, in fields that need the term, is often a [[measured figure]] specific to a sample. Therefore, they often vary since one is [[theoretical]] and the other is [[experimental]]. Specific samples may [[variation|vary]] significantly from the expected isotopic composition due to real [[deviation]]s from [[earth]]'s average isotopic abundances. == Computation == The molecular mass can be calculated as the sum of the [[individual]] [[isotopic mass]]es (as found in a [[table of isotopes]]) of all the atoms in any [[molecule]]. This is possible because molecules are created by [[chemical reaction]]s which, unlike [[nuclear reaction]]s, have very small [[binding energy|binding energies]] compared to the [[rest mass]] of the atoms (<math><</math> 10<sup>-9</sup>) and therefore create a negligible [[mass defect]]. The use of average [[atomic weight|atomic masses]] derived from the standard atomic weights found on a standard [[periodic table]] will result in an average molecular mass, whereas the use of isotopic masses will result in a molecular mass consistent with the strict interpretation of the definition, i.e. that of a single molecule. However, any given molecule may contain any given combination of isotopes, so there may be multiple molecular masses for each chemical compound. == Measurement == The molecular mass can also be measured directly using [[mass spectrometry]]. In mass spectrometry, the molecular mass of a small molecule is usually reported as the '''[[monoisotopic mass]]''', that is, the mass of the molecule containing only the most common isotope of each element. Note that this also differs subtly from the molecular mass in that the choice of isotopes is defined and thus is a single specific molecular mass of the many possible. The masses used to compute the monoisotopic molecular mass are found on a table of isotopic masses and are not found on a typical periodic table. The '''average molecular mass''' is often used for larger molecules since molecules with many atoms are unlikely to be composed exclusively of the most abundant isotope of each element. A theoretical average molecular mass can be calculated using the [[standard atomic weight]]s found on a typical periodic table, since there is likely to be a statistical distribution of atoms representing the isotopes throughout the molecule. This however may differ from the true average molecular mass of the sample due to natural (or artificial) variations in the isotopic distributions. == Unit Type Variation == The [[molar mass]] of a substance is the mass of 1&nbsp;mol (the SI unit for the basis SI quantity '''amount of substance''', having the symbol '''''n''''') of the substance. This has a numerical value which is the average molecular mass of the molecules in the substance multiplied by [[Avogadro's constant]] approximately 6.022*10<sup>23</sup>. The most common units of molar mass are ''g/mol'' because in those units the numerical value equals the average molecular mass in units of u. Conversion Factor of average molecular mass to molar mass: :'''molar mass = average molecular mass * ((1/6.022)*10<sup>-23</sup>g/u)*(6.022*10<sup>23</sup>/mol)''' :or :'''molar mass in g/mol= average molecular mass in u''' ''(Note that these relations are true for theoretical and experimental values, but not between experimental and theoretical values. Molar mass is most often theoretical and average molecular mass is most often experimental)'' The average atomic mass of natural [[hydrogen]] is 1.00794&nbsp;u and that of natural [[oxygen]] is 15.9994&nbsp;u; therefore, the molecular mass of natural [[water]] with formula H<sub>2</sub>O is (2 &times; 1.00794&nbsp;u) + 15.9994&nbsp;u = 18.01528&nbsp;u. Therefore, one [[mole (unit)|mole]] of water has a mass of 18.01528&nbsp;grams. However, the exact mass of hydrogen-1 (the most common hydrogen [[isotope]]) is 1.00783, and the exact mass of oxygen-16 (the most common oxygen isotope) is 15.9949, so the mass of the most common molecule of water is 18.01056&nbsp;u. The difference of 0.00472&nbsp;u or 0.03% comes from the fact that natural water contain traces of water molecules containing, oxygen-17, oxygen-18 or hydrogen-2 ([[Deuterium]]) atoms. Although this difference is trivial in bulk chemistry calculations, it can result in complete failure in situations where the behavior of individual molecules matters, such as in [[mass spectrometry]] and particle physics (where the mixture of isotopes does not act as an average). There are also situations where the isotopic distributions are not typical such as with [[heavy water]] used in some [[nuclear reactors]] which is artificially enriched with Deuterium. In these cases the computed values of molar mass and average molecular mass, which are ultimately derived from the [[standard atomic weight]]s, will not be the same as the actual molar mass or average molecular mass of the sample. In this case the mass of deuterium is 2.0136&nbsp;u and the average molecular mass of this water (assuming 100% deuterium enrichment) is (2 &times; 2.0136&nbsp;u) + 15.9994&nbsp;u = 20.0266&nbsp;u. This is a very large difference of ~11% error from the expected average molecular mass based on the standard atomic weights. Furthermore the most abundant molecular mass is actually slightly less than the average molecular mass since oxygen-16 is still the most common. (2 &times; 2.0136&nbsp;u) + 15.9949&nbsp;u = 20.0221&nbsp;u. Although this is an extreme artificial example, natural variation in isotopic distributions do occur and are measurable. == See also == * [[Avogadro's number]] * [[Atomic mass unit]] * [[Molar mass]] * [[Monoisotopic mass]] * [[gram-molecular weight]] == References == <div class="references-small"><references/></div> == External links == *[http://www.vias.org/simulations/simusoft_molform.html Learning by Simulations] Calculation of Molecular Formulas from Molecular Masses *[http://www.chemeurope.com/tools/mm.php3?language=e Molecular Mass Calculator] An online molecular mass calculator *[http://www.chemaxon.com/demosite/marvin/index.html Free online calculations for mol weight and elemental composition] using ChemAxon's Marvin and Calculator Plugins - requires Java <!--Categories--> [[Category:Mass]] [[Category:Chemical properties]] <!--Interwiki--> [[ar:كتلة جزيئية]] [[be:Адносная малекулярная маса]] [[bs:Relativna molekulska masa]] [[ca:Massa molecular]] [[de:Molekülmasse]] [[el:Μοριακό βάρος]] [[es:Masa molecular]] [[eo:Molekula maso]] [[eu:Masa molekular]] [[fa:وزن مولکولی]] [[fr:Masse moléculaire]] [[gl:Masa molecular]] [[ko:분자 질량]] [[hr:Relativna molekularna masa]] [[lv:Molekulu masa]] [[hu:Molekulatömeg]] [[nl:Moleculaire massa]] [[ja:分子量]] [[oc:Massa moleculara]] [[pl:Masa cząsteczkowa]] [[pt:Massa molecular]] [[ru:Молекулярная масса]] [[sk:Molekulová hmotnosť]] [[sr:Молекулска маса]] [[fi:Molekyylimassa]] [[sv:Molekylmassa]] [[uk:Грам-молекула]] [[zh:分子量]]