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Encyclopedia > Molar mass

Molar mass is the mass of one mole of a chemical element or chemical compound.[1] It is commonly used in stoichiometric calculations of bulk substances in chemistry. Its primary purpose is as a conversion factor between the number of grams of a pure substance, which can be measured directly, and the number of moles of that substance, which has greater chemical significance. This allows for using the appropriate number of molecules of a substance regardless of the mass. For example, if an equal number of molecules of two substances are needed for a reaction but the molar mass of one substance is twice that of the other, twice as many grams will be needed of that substance to give the same number of molecules. This article or section is in need of attention from an expert on the subject. ... The mole (symbol: mol) is the SI base unit that measures an amount of substance. ... The periodic table of the chemical elements A chemical element, or element, is a type of atom that is defined by its atomic number; that is, by the number of protons in its nucleus. ... A chemical compound is a chemical substance of two or more different chemically bonded chemical elements, with a fixed ratio determining the composition. ...


The molar mass of a chemical substance may be computed from the standard atomic weights listed for the elements on a standard periodic table. A mole of a substance is defined to be approximately 6.023x1023 (see Avogadro's number) of particles of the substance. Thus the molar mass is the mass of 6.023x1023 particles of the substance. Molar mass is different from Molecular mass which is the mass of one molecule. Water and steam are two different forms of the same chemical substance A chemical substance is any material with a definite chemical composition, no matter where it comes from. ... The atomic mass (ma) is the mass of an atom at rest, most often expressed in unified atomic mass units. ... “The Periodic Table” redirects here. ... Look up mole in Wiktionary, the free dictionary. ... 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 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). ...


In chemistry, the unit of molar mass is g/mol due to chemical utility. In physics, molar mass is usually defined in kilograms per mole (kg/mol) because the base SI unit of mass is the kilogram. For other uses, see Chemistry (disambiguation). ... A magnet levitating above a high-temperature superconductor demonstrates the Meissner effect. ... “Kg” redirects here. ...


In linear polymers not every polymer chain consists of the same amount of repeating units. A given polymer sample is said to be made up of a mixture of macromolecules with a certain molar mass distribution.[2] A polymer (from Greek: πολυ, polu, many; and μέρος, meros, part) is a substance composed of molecules with large molecular mass composed of repeating structural units, or monomers, connected by covalent chemical bonds. ... In polymer chemistry, a structural unit is a building block of a polymer chain. ... The Molar mass distribution in a polymer describes the relationship between a polymer fraction and the molar mass of that polymer fraction. ...

Contents

Example

Let us see approximately how many grams are in 2.3 moles of table sugar with a chemical formula of C12H22O11. The standard atomic weights of carbon, hydrogen, and oxygen are approximately 12.011 , 1.008 , and 15.999 g/mol respectively. Thus the molar mass of sucrose is its sum: (12.011 * 12) + (1.008 * 22) + (15.999 * 11) = 342.297 g/mol. The mass of 2.3 moles of sugar is then 2.3 mol * 342.297 g/mol = 787.2831 g. Sucrose (common name: table sugar, also called saccharose) is a disaccharide (glucose + fructose) with the molecular formula C12H22O11. ... The atomic mass (ma) is the mass of an atom at rest, most often expressed in unified atomic mass units. ...


(Note: For simplicity this example ignores the proper use of significant figures) Rounding to n significant figures is a form of rounding. ...


Molar mass versus molecular mass

Molar mass is sometimes confused with the related but distinct molecular mass. This is largely due to that when the molar mass and molecular mass are expressed in g/mol and u respectively they will almost always have similar but not identical numerical values. The molar mass is generally computed from isotopically weighted averages, whereas the molecular mass is the mass of a single molecule consisting of well-defined isotopes. The isotopically weighted averages used to compute molar masses are those found in most versions of the periodic table and are numbers recommended by IUPAC. They represent the most likely weights of substances found in the laboratory. The averaging takes into account the natural abundance of, usually heavier, isotopes as well as the variation in their natural abundance in different places on earth. Additionally the confidence, or number of significant figures after the decimal, is different. The significant figures in the standard atomic weights and thus the computed molar masses are often limited by the natural variations in the isotopic distributions and not necessarily by our ability to measure accurately. The confidence in the isotopic masses and resulting molecular masses are only limited by the accuracy of measurement of the invariable isotopic masses. 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). ... For other uses, see Isotope (disambiguation). ... The International Union of Pure and Applied Chemistry (IUPAC) is an international non-governmental organization devoted to the advancement of chemistry. ...


It is common, even amongst professional chemists, to use the terms interchangeably since for most common applications the difference is insignificant. This can, however, on occasion lead to substantive confusion. Due to this common practice some areas of chemistry have developed their own more specific terms such as monoisotopic mass and average mass. Due to these subtle differences and the inherent nature of the molar mass it is always more correct, accurate and consistent to use molar mass in any bulk stoichiometric calculations. 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. ...


References

  1. ^ Compendium of Chemical Terminology, relative molar mass
  2. ^ P. Stepan (2004). "Round Robin Test on the Molecular Characterization of Epoxy Resins by Liquid Chromatography". International Journal of Polymer Analysis and Characterization 9 (5): 305-316. DOI:10.1080/10236660490935718. 

Compendium of Chemical Terminology (ISBN 0-86542-684-8) is a book published by IUPAC containing internationally accepted definitions for terms in chemistry. ... A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ...

See also

The mole (symbol: mol) is the SI base unit that measures an amount of substance. ... The periodic table of the chemical elements A chemical element, or element, is a type of atom that is defined by its atomic number; that is, by the number of protons in its nucleus. ... 3D (left and center) and 2D (right) representations of the terpenoid molecule atisane. ... A chemical formula is a concise way of expressing information about the atoms that constitute a particular chemical compound. ... 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). ... 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. ...

External links

  • Molar Mass Calculator Online Molar Mass and Elemental Composition Calculator
  • Molecular Weight Calculator Online Molecular Weight Calculator
  • Molar Mass Calculator Freeware Windows Application

  Results from FactBites:
 
How to calculate the molar masses of chemical compounds (915 words)
Molar masses of common chemical compounds that you might find in the chemistry laboratory can range between 18 grams/mole for compounds like water to hundreds of grams per mole for more complex chemical compounds.
The molar mass of elements is found by looking at the atomic mass of the element on the periodic table.
Molar masses of chemical compounds are equal to the sums of the molar masses of all the atoms in one molecule of that compound.
  More results at FactBites »

 
 

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