Molecular weight is simply the weight of all the atoms in a substance. A molecule is a stable group of two or more atoms and is electrically neutral. Hydrogen and oxygen are atoms. Put two hydrogen and one oxygen together, and you get H₂O, or water, which is a molecule. The molecular weight of water is determined by adding together the weight of all three atoms in a single water molecule.

Molecular weight is also called molecular mass, and refers to the mass of one molecule of a substance, such as water. Molecular mass takes into account the average molecular weight, where the weight of one molecule may be slightly different than another because of isotopes. Isotopes are variations of elements and have a nuclei with the same number of protons but a different number of neutrons, giving them a different atomic weight that is sometimes figured into the calculations of molecular weight.

Some chemists refer to molecular weight or mass interchangeably with molar mass. Molar mass refers to the weight of one mole of a substance. A mole is the amount of a substance that contains the same number of elemental entities, such as electrons, ions, and atoms, as there are in 12 grams of carbon-12. Carbon-12 is the most stable of carbon isotopes, and is used as the standard by which all atomic masses are measured.

To determine the molecular weight of water, as an example, you would take the weight of hydrogen and multiply it by two, since there are two hydrogen atoms in water, and add the weight of oxygen. Hydrogen weighs about 1 atomic mass unit (u) and oxygen weighs about 16 u. To calculate the molecular weight of water, the equation is 1 × 2 + 16 = 18 u. The molecular weight of one molecule of water is 18 u.

Mass spectronomy can also be used to determine molecular mass. It is used to determine the weight of smaller molecules, while average molecular weight is used for larger molecules. Mass spectronomy uses the mass of the most common isotope of each element. Average molecular mass is calculated based on the most common isotope of an element, and is often calculated using the weights listed on the periodic chart, unless the exact isotope in the substance has been determined.