Molecular mass
The
molecular mass (abbreviated
MM) of a
substance, 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.
The molecular mass can be calculated as the sum of the
atomic masses of all the atoms of any one
molecule. 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 most common isotope of each element. 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.
The
molar mass of a substance is numerically equal to the molecular mass, but expressed in mass units per
mole, usually as
g/mol (
grams per mole).
Example:: The atomic mass of
hydrogen is 1.00784 u and that of
oxygen is 15.9994 u; therefore, the molecular mass of
water with formula H
2O is (2 × 1.00784 u) + 15.9994 u = 18.01508 u. Therefore, one mole of water weighs 18.0151 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 18.0105 u.
Molecular mass or molar mass are used in
stoichiometry calculations.
Since molecules are created by
chemical reactions, not
nuclear reactions, a molecule's molecular mass exactly equals the sum of the atomic masses of its constituent atoms.
The
gram-molecular weight is the
molecular weight of a chemical compound expressed in grams. Thus the molecular weight of
calcium carbonate is 100 therefore the gram-molecular weight is 100 g. The Gram Molecular Weight is frequently used, particularly for calculating the concentration of solutions. Thus a
molar solution will contain the gram molecular weight of a chemical dissolved to produce one litre of solution.
In
polymer chemistry, due to the varying chain lengths between the polymer macromolecules, various types of molecular mass are used to quantify the
molar mass distribution.
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Avogadro's number*
Atomic mass unit*
Learning by Simulations Calculation of Molecular Formulas from Molecular Masses
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Planet Chemistry Molar Mass Calculator
