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Encyclopedia > Molecular mass

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. A chemical compound is a chemical substance of two or more different chemically bonded chemical elements, with a fixed ratio determining the composition. ... Unsolved problems in physics: What causes anything to have mass? The U.S. National Prototype Kilogram, which currently serves as the primary standard for measuring mass in the U.S. Mass is the property of a physical object that quantifies the amount of matter and energy it is equivalent to. ... In science, a molecule is a group of atoms in a definite arrangement held together by chemical bonds. ... The atomic mass unit (amu), unified atomic mass unit (u), or dalton (Da), is a small unit of mass used to express atomic masses and molecular masses. ... Properties In chemistry and physics, an atom (Greek ἄτομος or átomos meaning indivisible) is the smallest particle still characterizing a chemical element. ... Carbon 12 is a stable isotope of the element carbon. ...


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. Molar mass is the mass of one mole of a chemical element or chemical compound. ... Isotopes are any of the several different forms of an element each having different atomic mass (mass number). ... Molar mass is the mass of one mole of a chemical element or chemical compound. ...

Contents

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. In science, a molecule is a group of atoms in a definite arrangement held together by chemical bonds. ... Vapours of hydrogen chloride in a beaker and ammonia in a test tube meet to form a cloud of a new substance, ammonium chloride A chemical reaction is a process that results in the interconversion of chemical substances. ... In nuclear physics, a nuclear reaction is a process in which two nuclei or nuclear particles collide to produce products different from the initial particles. ... ...


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. Mass spectrometry (also known as mass spectroscopy (deprecated)[1] or informally, mass-spec and MS) is an analytical technique used to measure the mass-to-charge ratio of ions. ... In mass spectrometry, monoisotopic mass is the sum of the exact or accurate masses of the lightest stable isotope of the atoms in a molecule. ...


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 . Molar mass is the mass of one mole of a chemical element or chemical compound. ...


Approximate Conversion Factor of molecular mass to molar mass


molar mass = molecular mass * (6.022*1023)

Example:
The atomic mass of hydrogen is 1.00794 u and that of oxygen is 15.9994 u;

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). General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... General Name, Symbol, Number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Standard atomic weight 15. ... Impact from a water drop causes an upward rebound jet surrounded by circular capillary waves. ... Look up mole in Wiktionary, the free dictionary. ...


See also

Avogadros number, also called Avogadros constant (NA), named after Amedeo Avogadro, is formally defined to be the number of carbon-12 atoms in 12 grams (0. ... The unified atomic mass unit (u), or Dalton (Da), is a small unit of mass used to express atomic and molecular masses. ... Molar mass is the mass of one mole of a chemical element or chemical compound. ... In mass spectrometry, monoisotopic mass is the sum of the exact or accurate masses of the lightest stable isotope of the atoms in a molecule. ... The mole (symbol: mol) is the SI base unit that measures an amount of substance. ...

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

  Results from FactBites:
 
Molecular Mass (875 words)
In theory, the relative molecular mass or molecular weight of a compound is the mass of a molecule of the compound relative to the mass of a carbon atom taken as exactly 12.
In practice, the molecular mass, MM, (molecular weight, MW) of a compound is the sum of the atomic masses (atomic weights) of the atomic species as given in the molecular formula.
Calculate the Molecular Mass (MM) of the compound carbon dioxide, CO The formula for carbon dioxide is composed of one atom of carbon and two atoms of oxygen
Molecular mass (184 words)
The molecular mass of a substance (less accurately called molecular weight and abbreviated as MW) is the mass of one molecule of that substance and is expressed in terms of the molecular mass unit u (equal to 1/12 the mass of one atom of carbon-12).
The molecular mass can be calculated as the sum of the atomic masses of all the atoms of any one molecule.
Strictly speaking, the molecular mass is not precisely equal to the sum of the atomic masses, but a little bit lower.
  More results at FactBites »

 
 

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