n. (Abbr. mol. wt.)
The sum of the atomic weights of all the atoms in a molecule. Also called formula weight.
molecular weight
| Dictionary: molecular weight |
n. (Abbr. mol. wt.)
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| Sci-Tech Encyclopedia: Molecular weight |
The sum of the atomic weights of all atoms making up a molecule. Actually, what is meant by molecular weight is molecular mass. The use of this expression is historical, however, and will be maintained. The atomic weight is the mass, in atomic mass units, of an atom. It is approximately equal to the total number of nucleons, protons and neutrons composing the nucleus. Since 1961 the official definition of the atomic mass unit (amu) has been that it is 1/12 the mass of the carbon-12 isotope, which is assigned the value 12.000 exactly. See also Atomic mass unit; Atomic mass unit; Relative atomic mass; Relative molecular mass.
A mole is an amount of substance containing the Avogadro number, NA, approximately 6.022 × 1023, of molecules or atoms. Molecule, in this definition, is understood to be the smallest unit making up the characteristic compound. Originally, the mole was interpreted as that number of particles whose total mass in grams was numerically equivalent to the atomic or molecular weight in atomic mass units, referred to as gram-atomic or gram-molecular weight. This is how the above value for NA was calculated. As the ability to make measurements of the absolute masses of single atoms and molecules has improved, however, modern metrology is tending to alter its approach and define the Avogadro number as an exact quantity, thereby changing slightly the definition of the atomic mass unit and removing the need to define atomic weight with respect to a particular isotopic species. The latest and most accurate value for the Avogadro number is 6.0221415(10) × 1023 mol−1. See also Avogadro number; Mole (chemistry).
As the masses of all the atomic species are now well known, masses of molecules can be determined once the composition of the molecule has been ascertained. Alternatively, if the molecular weight of the molecule is known and enough additional information about composition is available, such as the basic atomic constituents, it is possible to begin to assemble structural information about the molecule. Thus, the determination of the molecular weight is one of the first steps in the analysis of an unknown species. Given the increasing emphasis on the study of biologically important molecules, particular attention has been focused on the determination of molecular weights of larger and larger units. There are a number of methods available, and the one chosen will depend on the size and physical state of the molecule. All processes are physical macroscopic measurements and determine the molecular weight directly. Connection to the absolute mass scale is straightforward by using the Avogadro number, although, for extremely large molecules, this connection is often unnecessary or impossible, as the accuracy of the measurements is not that good. The main function of molecular weight determination of large molecules is elucidation of structure.
Molecular weight determination of materials which are solid or liquid at room temperature is best achieved by taking advantage of one of the colligative properties of solutions, boiling-point elevation, freezing-point lowering, or osmotic pressure, which depend on the number of particles in solution, not on the nature of the particle. The choice of which to use will depend on a number of properties of the substance, the most important of which will be the size. All require that the molecule be small enough to dissolve in the solution but large enough not to participate in the phase change or pass through a semipermeable membrane. Freezing-point lowering is an excellent method for determining molecular weights of smaller organic molecules, and osmometry, as the osmotic pressure determination is called, for determining molecular weights of larger organic molecules, particularly polymeric species. Boiling-point elevation is used less frequently. See also Polymer.
The basis of all the methods involving colligative properties of solutions is that the chemical potentials of all phases must be the same. (Chemical potential is the partial change in energy of a system as matter is transferred into or out of it. For two systems in contact at equilibrium, the chemical potentials for each must be equal.) See also
Another measurement from which molecular weights can be obtained is based on the scattering of light from the molecule. A beam of light falling on a molecule will induce in the molecule a dipole moment which in its turn will radiate. The interference between the radiated beam and the incoming beam produces an angular dependence of the scattered radiation which depends on the molecular weight of the molecule. This occurs whether the molecule is free or in solution. While the theory for this effect is complicated and varies according to the size of the molecule, the general result for molecules whose size is considerably less than that of the wavelength λ of the radiation (less than λ/50) is given by the equation below; 
I(θ) is the intensity of radiation at angle θ, I0 the intensity of the incoming beam, M the molecular weight, and c the concentration in grams per cubic centimeter of the molecule. If the molecules are much larger than λ/50 (about 9 nanometers for visible light), this relationship in this simple form is no longer valid, but the method is still viable with appropriate adjustments to the theory. In fact, it can be used in its extended version even for large aggregates.
| Dental Dictionary: molecular weight |
(məlek′yōōlur)
n
n
The sum of the atomic weights of all atoms in a molecule.
| Britannica Concise Encyclopedia: molecular weight |
For more information on molecular weight, visit Britannica.com.
| Columbia Encyclopedia: molecular weight |
molecular weight, weight of a molecule of a substance expressed in atomic mass units (amu). The molecular weight may be calculated from the molecular formula of the substance; it is the sum of the atomic weights of the atoms making up the molecule. For example, water has the molecular formula H2O, indicating that there are two atoms of hydrogen and one atom of oxygen in a molecule of water. Rounded to three decimal places, the atomic weight of hydrogen is 1.008 amu and that of oxygen is 15.999 amu. The molecular weight of water is thus (2×1.008)+(1×15.999)=2.016+15.999=18.015 amu. Since atomic weights are average values, molecular weights are also average values. On the average, a molecule of ordinary water weighs 18.015 amu. Both hydrogen and oxygen are made up of several isotopes. One isotope of hydrogen is deuterium, or heavy hydrogen. Atoms of deuterium are about twice as massive as the average for all hydrogen atoms in ordinary water. Therefore water that contains only atoms of deuterium, called heavy water, has a higher molecular weight than ordinary water. Some substances, especially ionic compounds such as common salt, are not made up of molecules and thus have neither a molecular formula nor a molecular weight.
Molecular weights of substances may be determined experimentally in various ways, the method employed usually depending on the state (solid, liquid, or gas) of the substance. Methods for determining the molecular weights of gaseous substances are based on Avogadro's law, which states that under given conditions of temperature and pressure a given volume of any gas contains a specific number of molecules of the gas; thus a comparison of the weights of equal volumes of different gases under the same conditions of temperature and pressure is equivalent to a direct comparison of the weights of molecules of the gases. The molecular weights of substances that are not normally gaseous and do not evaporate without decomposition are sometimes determined from their effects on the melting point, boiling point, vapor pressure, or osmotic pressure of some solvent (see colligative properties). However, if the substance ionizes or does not completely separate into molecules, the molecular weight so determined will be erroneous. Highly accurate molecular weights are sometimes determined by using the mass spectrograph.
Some substances, e.g., proteins, viruses, and certain synthetic polymers, have very high molecular weights. These molecular weights may be determined by measurement of sedimentation rate in an ultracentrifuge, by light-scattering photometry, or by other methods. The methods may give different results, since usually the molecules of a substance such as a polymer do not all have exactly the same molecular weight. These methods determine an average molecular weight for the molecules in the sample. The number-average molecular weight determined by the ultracentrifuge method gives a value that is equal to the weight of the sample divided by the number of molecules in the sample. This number-average molecular weight can also be determined by other methods based on measurement of colligative properties. The light-scattering method determines what is called the weight-average molecular weight. Although this may be the same value as the number-average molecular weight if all the molecules have nearly the same weight, it will be higher if some of the molecules are heavier than others.
| Science Dictionary: molecular weight |
(muh-lek-yuh-luhr)
The sum of the atomic weights of all the atoms in a molecule.
| Wikipedia: Molecular mass |
This article is about the mass of a single molecule. For the mass of a mole of a substance commonly used in basic stoichiometric calculations, see Molar mass.
The molecular mass (abbreviated M) of a substance, frequently referred by the older term molecular weight and abbreviated as MW, is the mass of one molecule of that substance, relative to the unified atomic mass unit u[1] (equal to 1/12 the mass of one isotope of carbon-12[2]). 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 common measurements of the mass of chemicals, such as molar mass, by taking into account the isotopic composition of a molecule rather than the average isotopic distribution of many molecules. As a result, molecular mass is a more precise number than molar mass; however it is more accurate to use molar mass on bulk samples. This means that molar mass is appropriate most of the time except when dealing with single molecules.
Contents |
Definition
There are varying interpretations of this definition. Many chemists use molecular mass as a synonym of molar mass,[3] 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 isotopes, 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 error becomes much larger when considering larger molecules or less 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 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 weights, 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 vary significantly from the expected isotopic composition due to real deviations from earth's average isotopic abundances.
Computation
The molecular mass can be calculated as the sum of the individual isotopic masses (as found in a table of isotopes) of all the atoms in any molecule. This is possible because molecules are created by chemical reactions which, unlike nuclear reactions, have very small binding energies compared to the rest mass of the atoms ( < 10-9) and therefore create a negligible mass defect. The use of average 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 weights 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 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*1023. 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)*10-23g/u)*(6.022*1023/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 u and that of natural oxygen is 15.9994 u; therefore, the molecular mass of natural water with formula H2O is (2 × 1.00794 u) + 15.9994 u = 18.01528 u. Therefore, one mole of water has a mass of 18.01528 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 u. The difference of 0.00472 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 weights, 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 u and the average molecular mass of this water (assuming 100% deuterium enrichment) is (2 × 2.0136 u) + 15.9994 u = 20.0266 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 × 2.0136 u) + 15.9949 u = 20.0221 u. Although this is an extreme artificial example, natural variation in isotopic distributions do occur and are measurable. For example, the atomic weight of lithium as found by isotopic analysis of 39 lithium reagents from several manufacturers varied from 6.939 to 6.996.[4]
See also
Notes
External links
- Learning by Simulations Calculation of Molecular Formulas from Molecular Masses
- Molecular Mass Calculator An online molecular mass calculator
- Free online calculations for mol weight and elemental composition using ChemAxon's Marvin and Calculator Plugins - requires Java
- Molecular Mass Calculator Online Molecular Mass and Elemental Composition Calculator
- Calculate a molecular formula:ChemCalc Online calculation of Molecular Mass with isotopic distribution
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Mentioned in
- mol. wt. (abbreviation)
- Fitzgerald factor
- Flaujeac factor
- kilodalton
- gram-molecular weight (chemistry)
- molecular volume (chemistry)
- monodispersity (organic chemistry)
- polydispersity (chemistry)
- formula weight (chemistry)
- ribosomal ribonucleic acid (biochemistry)
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