Molecular mass
From Wikipedia, the free encyclopedia
The molecular mass (abbreviated Mr) of a substance, formerly also called molecular weight and abbreviated as MW, is the mass of one molecule of that substance, relative to the unified atomic mass unit u (equal to 1/12 the mass of one atom of carbon-12). Due to this relativity, the molecular mass of a substance is commonly referred to as the relative molecular mass, and abbreviated to Mr.
Although this term appears well-defined there are varying interpretations of this definition. It is interpreted by many, including many chemists, to be a synonym of molar mass differing only in units. This is inconsistent with a strict interpretation of the definition because it neglects that the mass of a single molecule is not the same as the average of an ensemble. A mole of molecules most often contains a variety of molecular masses due to natural isotopes and the average is usually not identical to any single molecule. The actual difference numerically is very small and 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.
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[edit] Computing the Molecular Mass
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 one molecule. This is possible because molecules are created by chemical reactions, not nuclear reactions and therefore there is no mass defect created. Note that the use of average atomic masses as 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. Note that any given molecule may contain any given combination of isotopes, so there may be multiple molecular masses for each chemical compound.
[edit] Measuring the Molecular Mass
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. The masses used to compute the monoisotopic molecular mass are found on a table of isotopic masses and are not the same as 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. This average mass can be calculated using the elemental mass numbers on a typical periodic table, since there is likely to be a statistical distribution of atoms representing the isotopes throughout the molecule.
[edit] Molecular Mass versus Molar Mass
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). This has a numerical value which is approximately the molecular mass multiplied by Avogadro's constant 6.022*1023, and the SI unit kg/mol, usually also found as stated in g/mol .
Approximate Conversion Factor of molecular mass to molar mass
molar mass = molecular mass * (6.022*1023)
therefore, the molecular mass of 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 isotope) is 1.00783, and the exact mass of oxygen-16 (the most common isotope) is 15.9949, so the mass of the most common single molecule of water is 18.01056 u. This is a difference of 0.00472 u or 0.03%. Although this difference is trivial in bulk calculations, it can result in complete failure in situations where the behavior of individual molecules matters, such as in particle physics (where the mixture of isotopes does not act as an average).
[edit] See also
[edit] External links
- Learning by Simulations Calculation of Molecular Formulas from Molecular Masses
- Molecular Mass Calculator An online molecular mass calculator
- Molecular Weight Calculator Calculates molecular weight and elemental composition
- Free online calculations for mol weight and elemental composition using ChemAxon's Marvin and Calculator Plugins - requires Java
