# Molecular Weight

on . Posted in Thermodynamics

Molecular weight, abbreviated as MW, also called molecular mass, is the mass of a molecule relative to the unified atomic mass unit or the gram per mole.  It is a fundamental property of a chemical compound and is calculated by summing the atomic masses of all the atoms in a molecule.  The molecular weight is used to determine the amount of substance, in moles, present in a given mass of a substance.  It is also crucial for various calculations in chemistry, such as stoichiometry, determining empirical and molecular formulas, and converting between mass and moles.  It is important to note that the molecular weight is an average value, as different isotopes of an element may exist, each with a slightly different mass.

### Molecular weight formula

$$MW = R^* \;/\; R$$     (Molecular Weight)

$$R^* = MW \; R$$

$$R = R^* \;/\; MW$$

Symbol English Metric
$$MW$$ = molecular weight $$lbm \;/\; lbmol$$     $$kg \;/\; kmol$$
$$R^*$$ = universal gas constant $$lbf-ft \;/\; lbmol-R$$ $$J \;/\; kmol-K$$
$$R$$ = specific gas constant $$ft-lbf \;/\; lbm-R$$  $$J \;/\; kg-K$$