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of contents molar mass 75 languages afrikaans aragonés العربية azərbaycanca تۆرکجه беларуская тарашкевіца беларуская български bosanski català čeština чӑвашла dansk deutsch ελληνικά esperanto español eesti euskara فارسی suomi français galego עברית हिन्दी hrvatski kreyòl ayisyen magyar հայերեն interlingua bahasa indonesia italiano 日本語 ქართული қазақша 한국어 кыргызча latina lietuvių latviešu мокшень македонски മലയാളം монгол bahasa melayu nederlands norsk nynorsk norsk bokmål occitan ਪੰਜਾਬੀ polski português română русский davvisámegiella srpskohrvatski српскохрватски simple english slovenčina slovenščina shqip српски srpski svenska தமிழ் తెలుగు ไทย tagalog türkçe українська اردو oʻzbekcha ўзбекча tiếng việt 吴语 文言 粵語 中文 edit links article talk english read edit view history tools tools move to sidebar hide actions read edit view history general what links here related changes upload file permanent link page information cite this page get shortened url switch to legacy parser print export download as pdf printable version in other projects wikidata item appearance move to sidebar hide from wikipedia the free encyclopedia mass per amount of substance not to be confused with molecular mass or mass number molar masses a diagram comparing moles and molar masses of iron and gold samples that have equal masses common symbols m si unit kg mol other units g mol dimension m n 1 in chemistry the molar mass m sometimes called molecular weight or formula weight but see related quantities for usage of a chemical substance element or compound is defined as the ratio between the mass m and the amount of substance n measured in moles of any sample of the substance m m n 1 the molar mass is a bulk not molecular property of a substance the molar mass is a weighted average of many instances of the element or compound which often vary in mass due to the presence of isotopes most commonly the molar mass is computed from the standard atomic weights and is thus a terrestrial average and a function of the relative abundance of the isotopes of the constituent atoms on earth the molecular mass for molecular compounds and formula mass for non molecular compounds such as ionic salts are commonly used as synonyms of molar mass as the numerical values are identical for all practical purposes differing only in units dalton vs g mol or kg kmol however the most authoritative sources define it differently the difference is that molecular mass is the mass of one specific particle or molecule a microscopic quantity while the molar mass is an average over many particles or molecules a macroscopic quantity the molar mass is an intensive property of the substance that does not depend on the size of the sample in the international system of units si the coherent unit of molar mass is kg mol however for historical reasons molar masses are almost always expressed with the unit g mol or equivalently in kg kmol since 1971 si defined the amount of substance as a separate dimension of measurement until 2019 the mole was defined as the amount of substance that has as many constituent particles as there are atoms in 12 grams of carbon 12 with the dalton defined as 1 12 of the mass of a carbon 12 atom thus during that period the numerical value of the molar mass of a substance expressed in g mol was exactly equal to the numerical value of the average mass of an entity atom molecule formula unit of the substance expressed in daltons since 2019 the mole has been redefined in the si as the amount of any substance containing exactly 6 022 140 76 10 23 entities fixing the numerical value of the avogadro constant n a when expressed in the unit mol 1 but because the dalton is still defined in terms of the experimentally determined mass of a carbon 12 atom the numerical equivalence between the molar mass of a substance and the average mass of an entity of the substance is now only approximate but equality may still be assumed with high accuracy the relative discrepancy is only of order 10 9 i e within a part per billion technical background edit for a pure sample of a substance x the known molar mass m x is used for calculating the amount of the substance in the sample n x given the mass of the sample m x through the equation n x m x m x if n x is the number of entities of the substance in the sample and m a x is the mass of each entity of the substance atomic mass molecular mass or formula mass then the mass of the sample is m x n x m a x and the amount of substance is n x n x n a n x n a where n a is the elementary amount an amount consisting of exactly one atomic scale entity of any kind atom molecule formula unit analogous to the elementary charge e since the elementary amount is the reciprocal of the avogadro constant using the relationship m x m x n x the molar mass is then given by m x m a x n a m a x n a dimension m n i e the atomic scale mass of one entity of the substance per elementary amount given the relative atomic scale mass atomic weight molecular weight or formula weight a r x of an entity of a substance x its mass expressed in daltons is m a x a r x da where the atomic scale unit of mass is defined as 1 da m u m a 12 c 12 dimension m the corresponding atomic scale unit of amount of substance is the entity symbol ent defined as 1 ent n a dimension n so with a r x known the molar mass can be expressed in daltons per entity as m x a r x da ent thus the molar mass of a substance x can be calculated as m x a r x m u with the molar mass constant m u equal to exactly 1 da ent which for all practical purposes is equal to 1 g mol as the mole was historically defined such that the avogadro number the number of atomic scale entities comprising one mole was exactly equal to the number of daltons in a gram g da this means that for all practical purposes 1 mol g da ent the relationship between the molar mass of carbon 12 m 12 c 12 g mol and its atomic mass m a 12 c 12 da can be expressed as m 12 c m a 12 c n a rearranging and substituting the given values into the equation yields the following expression for the avogadro constant n a g da mol 1 making the avogadro number equal to the number of daltons in a gram and equivalently the number of atoms in 12 grams of carbon 12 as in the 1971 definition of the mole the mole was defined in such a way that the numerical value of the molar mass of a substance in g mol i e m x g mol was equal to the numerical value of the average mass of one entity atom molecule formula unit in da i e m a x da a r x so that m x a r x g mol the equivalence was exact before the redefinition of the mole in 2019 and is now only approximate but equality may still be assumed with high accuracy thus for example the average mass of a molecule of water is about 18 0153 da and the molar mass of water is about 18 0153 g mol for chemical elements without isolated molecules such as carbon and metals the molar mass is calculated using the relative atomic mass of the element usually given by the standard atomic weight indicated in the periodic table thus for example the molar mass of iron is about 55 845 g mol calculation edit molar masses of elements edit main articles relative atomic mass and standard atomic weight the molar mass m x of atoms of an element x is given by the relative atomic mass a r x of the element multiplied by the molar mass constant m u which for all practical purposes is equal to 1 g mol m x a r x m u for normal samples from earth with typical isotope composition the atomic weight can be approximated by the standard atomic weight 2 or the conventional atomic weight m he 4 002602 2 m u 4 002602 2 g mol m ne 20 1797 6 m u 20 1797 6 g mol m fe 55 845 2 m u 55 845 2 g mol m cu 63 546 3 m u 63 546 3 g mol m ag 107 8682 2 m u 107 8682 2 g mol displaystyle begin array lll m ce he 4 002602 2 times m_ mathrm u 4 002602 2 text g mol m ce ne 20 1797 6 times m_ mathrm u 20 1797 6 text g mol m ce fe 55 845 2 times m_ mathrm u 55 845 2 text g mol m ce cu 63 546 3 times m_ mathrm u 63 546 3 text g mol m ce ag 107 8682 2 times m_ mathrm u 107 8682 2 text g mol end array multiplying by the molar mass constant ensures that the calculation is dimensionally correct relative atomic masses and standard atomic weights are dimensionless quantities i e pure numbers whereas molar masses have units in this case grams per mole some elements are usually encountered as molecules e g hydrogen h 2 nitrogen n 2 oxygen o 2 sulfur s 8 chlorine cl 2 the molar mass of molecules of these elements is the molar mass of the atoms multiplied by the number of atoms in each molecule m h 2 2 1 00794 7 m u 2 01588 14 g mol m n 2 2 14 0067 2 m u 28 0134 4 g mol m o 2 2 15 9994 3 m u 31 9988 6 g mol m s 8 8 32 065 5 m u 256 52 4 g mol m cl 2 2 35 453 2 m u 70 906 4 g mol displaystyle begin array lll m ce h2 2 times 1 00794 7 times m_ mathrm u 2 01588 14 text g mol m ce n2 2 times 14 0067 2 times m_ mathrm u 28 0134 4 text g mol m ce o2 2 times 15 9994 3 times m_ mathrm u 31 9988 6 text g mol m ce s8 8 times 32 065 5 times m_ mathrm u 256 52 4 text g mol m ce cl2 2 times 35 453 2 times m_ mathrm u 70 906 4 text g mol end array molar masses of compounds edit main article molecular mass calculation the molar mass m x of a compound is given by the sum of the relative atomic masses a r x i of the elements each multiplied by the number of atoms n i per element which form the compound multiplied by the molar mass constant m u 1 g mol m x m r x m u m u i n i a r x i displaystyle m text x m_ text r text x cdot m_ text u m_ text u sum _ i n_ i a_ text r text x _ i here m r x is the relative molar mass also called molecular weight or 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 m nacl 22 98976928 2 35 453 2 m u 58 443 2 g mol m c 12 h 22 o 11 12 12 0107 8 22 1 00794 7 11 15 9994 3 m u 342 297 14 g mol displaystyle begin array lll m ce nacl 22 98976928 2 35 453 2 times m_ text u 58 443 2 text g mol m ce c12h22o11 12 times 12 0107 8 22 times 1 00794 7 11 times 15 9994 3 times m_ text u 342 297 14 text g mol end array average molar mass of mixtures edit an average molar mass may be defined for mixtures of substances 1 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 3 4 the average molar mass of mixtures m displaystyle overline m can be calculated from the mole fractions x i of the components and their molar masses m i m i x i m i displaystyle overline m sum _ i x_ i m_ i it can also be calculated from the mass fractions w i of the components 1 m i w i m i displaystyle frac 1 overline m sum _ i frac w_ i m_ i as an example the average molar mass of dry air is 28 9647 g mol 5 related quantities edit molar mass is closely related to the molecular weight m w for molecular compounds and formula weight f w for non molecular compounds older terms for what is now more correctly called the relative molar mass m r 6 a dimensionless quantity i e a pure number without units equal to the molar mass divided by the molar mass constant calculated from the standard atomic weights of its constituent elements however it should be distinguished from the molecular mass which is confusingly also sometimes known as molecular weight which is the mass of one molecule of any single isotopic composition and to the atomic mass which is the mass of one atom of any single isotope the dalton symbol da is also sometimes used as a unit of molecular weight and formula weight now called relative molar mass especially in biochemistry despite the fact that the quantities are dimensionless as relative masses obsolete terms for molar mass include gram atomic mass for the mass in grams of one mole of atoms of an element and gram molecular mass for the mass in grams of one mole of molecules of a compound the gram atom is a former term for a mole of atoms and gram molecule for a mole of molecules 7 molecular mass edit main article molecular mass the molecular mass m is the mass of a given molecule it is usually measured in daltons da or u 7 different molecules of the same compound may have different molecular masses because they contain different isotopes of an element this is distinct but related to the molar mass which is a measure of the average molecular mass of all the molecules in a sample and is usually the more appropriate measure when dealing with macroscopic weigh able quantities of a substance molecular masses are calculated from the atomic masses of each nuclide while molar masses are calculated from the standard atomic weights 8 of each element the standard atomic weight takes into account the isotopic distribution of the element in a given sample usually assumed to be normal for example water has a molar mass of 18 0153 3 g mol but individual water molecules have molecular masses which range between 18 010 564 6863 15 da 1 h 2 16 o and 22 027 7364 9 da 2 h 2 18 o the distinction between molar mass and molecular mass is important because relative molecular masses can be measured directly by mass spectrometry often to a precision of a few parts per million this is accurate enough to directly determine the chemical formula of a molecule 9 dna synthesis usage edit the term formula weight has a specific meaning when used in the context of dna synthesis whereas an individual phosphoramidite nucleobase to be added to a dna polymer has protecting groups and has its molecular weight quoted including these groups the amount of molecular weight that is ultimately added by this nucleobase to a dna polymer is referred to as the nucleobase s formula weight i e the molecular weight of this nucleobase within the dna polymer minus protecting groups citation needed precision and uncertainties edit the precision to which a molar mass is known depends on the precision of the atomic masses 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 most atomic masses are known to a precision of at least one part in ten thousand often much better 2 the atomic mass of lithium is a notable and serious 10 exception this is adequate for almost all normal uses in chemistry it is more precise than most 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 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 s...