Molecular mass

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Not to be confused with Molar mass or Mass number. Molecular mass_sentence_0

The molecular mass (m) is the mass of a given molecule: it is measured in daltons (Da or u). Molecular mass_sentence_1

Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element. Molecular mass_sentence_2

The related quantity relative molecular mass, as defined by IUPAC, is the ratio of the mass of a molecule to the unified atomic mass unit (also known as the dalton) and is unitless. Molecular mass_sentence_3

The molecular mass and relative molecular mass are distinct from but related to the molar mass. Molecular mass_sentence_4

The molar mass is defined as the mass of a given substance divided by the amount of a substance and is expressed in g/mol. Molecular mass_sentence_5

The molar mass is usually the more appropriate figure when dealing with macroscopic (weigh-able) quantities of a substance. Molecular mass_sentence_6

The definition of molecular weight is most authoritatively synonymous with molecular mass; however, in common practice, it is also highly variable as are the units used in conjunction with it. Molecular mass_sentence_7

Many common preparatory sources use g/mol and effectively define it as a synonym of molar mass, while more authoritative sources use Da or u and align its definition more closely with the molecular mass. Molecular mass_sentence_8

Even when the molecular weight is used with the units Da or u, it is frequently as a weighted average similar to the molar mass but with different units. Molecular mass_sentence_9

In molecular biology, the weight of macromolecules is referred to as their molecular weight and is expressed in kDa, although the numerical value is often approximate and representative of an average. Molecular mass_sentence_10

The terms molecular mass, molecular weight, and molar mass are often used interchangeably in areas of science where distinguishing between them is unhelpful. Molecular mass_sentence_11

In other areas of science, the distinction is crucial. Molecular mass_sentence_12

The molecular mass is more commonly used when referring to the mass of a single or specific well-defined molecule and less commonly than molecular weight when referring to a weighted average of a sample. Molecular mass_sentence_13

Prior to the 2019 redefinition of SI base units quantities expressed in daltons (Da or u) were by definition numerically equivalent to otherwise identical quantities expressed in the units g/mol and were thus strictly numerically interchangeable. Molecular mass_sentence_14

After the 20 May 2019 redefinition of units, this relationship is only nearly equivalent. Molecular mass_sentence_15

The molecular mass of small to medium size molecules, measured by mass spectrometry, can be used to determine the composition of elements in the molecule. Molecular mass_sentence_16

The molecular masses of macromolecules, such as proteins, can also be determined by mass spectrometry; however, methods based on viscosity and light-scattering are also used to determine molecular mass when crystallographic or mass spectrometric data are not available. Molecular mass_sentence_17

Calculation Molecular mass_section_0

Molecular masses are calculated from the atomic masses of each nuclide present in the molecule, while molar masses are calculated from the standard atomic weights of each element. Molecular mass_sentence_18

The standard atomic weight takes into account the isotopic distribution of the element in a given sample (usually assumed to be "normal"). Molecular mass_sentence_19

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 (H 2O) and 22.027 7364(9) Da (H 2O). Molecular mass_sentence_20

Atomic and molecular masses are usually reported in daltons which is defined relative to the mass of the isotope C (carbon 12), which by definition is equal to 12 Da. Molecular mass_sentence_21

For example, the molar mass and molecular mass of methane, whose molecular formula is CH4, are calculated respectively as follows: Molecular mass_sentence_22

Molecular mass_table_general_0

Molar mass or molecular weight of CH4Molecular mass_header_cell_0_0_0
Molecular mass_header_cell_0_1_0 Standard atomic weightMolecular mass_header_cell_0_1_1 NumberMolecular mass_header_cell_0_1_2 Total molar mass (g/mol)

or molecular weight (Da or g/mol)Molecular mass_header_cell_0_1_3

CMolecular mass_cell_0_2_0 12.011Molecular mass_cell_0_2_1 1Molecular mass_cell_0_2_2 12.011Molecular mass_cell_0_2_3
HMolecular mass_cell_0_3_0 1.008Molecular mass_cell_0_3_1 4Molecular mass_cell_0_3_2 4.032Molecular mass_cell_0_3_3
CH4Molecular mass_cell_0_4_0 Molecular mass_cell_0_4_1 Molecular mass_cell_0_4_2 16.043Molecular mass_cell_0_4_3
Molecular mass of CH4Molecular mass_header_cell_0_5_0
Molecular mass_header_cell_0_6_0 Nuclide massMolecular mass_header_cell_0_6_1 NumberMolecular mass_header_cell_0_6_2 Total molecular mass (Da or u)Molecular mass_header_cell_0_6_3
CMolecular mass_cell_0_7_0 12.00Molecular mass_cell_0_7_1 1Molecular mass_cell_0_7_2 12.00Molecular mass_cell_0_7_3
HMolecular mass_cell_0_8_0 1.007825Molecular mass_cell_0_8_1 4Molecular mass_cell_0_8_2 4.0313Molecular mass_cell_0_8_3
CH4Molecular mass_cell_0_9_0 Molecular mass_cell_0_9_1 Molecular mass_cell_0_9_2 16.0313Molecular mass_cell_0_9_3

The more formally defined term is "relative molecular mass". Molecular mass_sentence_23

Relative atomic and molecular mass values as defined are dimensionless. Molecular mass_sentence_24

However, the adjective 'relative' is omitted in practice as it is universally assumed that atomic and molecular masses are relative to the mass of C. Additionally, the "unit" Dalton is used in common practice. Molecular mass_sentence_25

The mass of 1 mol of substance is designated as molar mass. Molecular mass_sentence_26

By definition, the molar mass has the units of grams per mole. Molecular mass_sentence_27

In the example above the standard atomic weight of carbon is 12.011 g/mol, not 12.00 g/mol. Molecular mass_sentence_28

This is because naturally occurring carbon is a mixture of the isotopes C, C and C which have masses of 12 Da, 13.003355 Da, and 14.003242 Da respectively. Molecular mass_sentence_29

Moreover, the proportion of the isotopes varies between samples, so 12.011 g/mol is an average value across different places on earth. Molecular mass_sentence_30

By contrast, there is less variation in naturally occurring hydrogen so the standard atomic weight has less variance. Molecular mass_sentence_31

The precision of the molar mass is limited by the highest variance standard atomic weight, in this example that of carbon. Molecular mass_sentence_32

This uncertainty is not the same as the uncertainty in the molecular mass, which reflects variance (error) in measurement not the natural variance in isotopic abundances across the globe. Molecular mass_sentence_33

In high-resolution mass spectrometry the mass isotopomers CH4 and CH4 are observed as distinct molecules, with molecular masses of approximately 16.031 Da and 17.035 Da, respectively. Molecular mass_sentence_34

The intensity of the mass-spectrometry peaks is proportional to the isotopic abundances in the molecular species. Molecular mass_sentence_35

C H H3 can also be observed with molecular mass of 17 Da. Molecular mass_sentence_36

Determination Molecular mass_section_1

Mass spectrometry Molecular mass_section_2

Main article: Mass spectrometry Molecular mass_sentence_37

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. Molecular mass_sentence_38

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 possibilities. Molecular mass_sentence_39

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. Molecular mass_sentence_40

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. Molecular mass_sentence_41

A theoretical average molecular mass can be calculated using the standard atomic weights found on a typical periodic table, since there is likely to be a statistical distribution of atoms representing the isotopes throughout the molecule. Molecular mass_sentence_42

The average molecular mass of a sample, however, usually differs substantially from this since a single sample average is not the same as the average of many geographically distributed samples. Molecular mass_sentence_43

Hydrodynamic methods Molecular mass_section_3

To a first approximation, the basis for determination of molecular mass according to Mark–Houwink relations is the fact that the intrinsic viscosity of solutions (or suspensions) of macromolecules depends on volumetric proportion of the dispersed particles in a particular solvent. Molecular mass_sentence_44

Specifically, the hydrodynamic size as related to molecular mass depends on a conversion factor, describing the shape of a particular molecule. Molecular mass_sentence_45

This allows the apparent molecular mass to be described from a range of techniques sensitive to hydrodynamic effects, including DLS, SEC (also known as GPC when the eluent is an organic solvent), viscometry, and diffusion ordered nuclear magnetic resonance spectroscopy (DOSY). Molecular mass_sentence_46

The apparent hydrodynamic size can then be used to approximate molecular mass using a series of macromolecule-specific standards. Molecular mass_sentence_47

As this requires calibration, it's frequently described as a "relative" molecular mass determination method. Molecular mass_sentence_48

Static light scattering Molecular mass_section_4

It is also possible to determine absolute molecular mass directly from light scattering, traditionally using the Zimm method. Molecular mass_sentence_49

This can be accomplished either via classical static light scattering or via multi-angle light scattering detectors. Molecular mass_sentence_50

Molecular masses determined by this method do not require calibration, hence the term "absolute". Molecular mass_sentence_51

The only external measurement required is refractive index increment, which describes the change in refractive index with concentration. Molecular mass_sentence_52

See also Molecular mass_section_5

Molecular mass_unordered_list_0


Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Molecular mass.