The molecular weight of a molecule is the sum of the atomic weights of all the atoms in the molecule. The periodic table of the elements lists the atomic weight of each atom based on carbon. The subscripts in the molecular formula represent the number of atoms of each element in the molecule.
When chemists calculate molecular weights, they usually round the numbers to two places after the decimal point. With larger molecules, they calculate the molecular weight to four significant figures.
The molecular weight of a compound is the weight in grams of one mole of the molecule. Chemists use the term "mole" as a collective term to describe Avogadro's number, 6.023x10^23, of atoms or molecules. Hydrogen, which exists in nature as H2, has a molecular weight of 2.02 grams per mole. Long carbon chain compounds have molecular weights of hundreds of grams. Some polymers have molecular weights that are over 1 million grams.
The molecular weight of a compound serves as a conversion factor between moles and grams. If an experiment requires a given number of moles of a compound, multiplying the number given by the molecular weight in grams per mole produces the number of grams the experiment requires, while multiplying the number of grams by the inverse of the molecular weight gives the number of moles.