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 M_r.

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 H₂O 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.

Molecular Mass in Polymer Chemistry

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.

---

This page includes material from the Wikipedia article "Molecular Mass"
Text between the lines is available under the GNU Free Documentation License.