Molecular weight refers to the weight or mass of a particular molecule. Substances are basically composed of molecules and their total mass represents their molecular weight. Molecular weight and molecular mass are terms that can be used interchangeably. For scientists, determining the molecular weight of a substance can help them study other properties of substances like volume, density, and liquid displacement among many others. Knowing the molecular weight also enables scientists to know the possible reaction of one substance when mixed with other substances.
The molecules in substances are composed of specific atoms. This simply means that the sum of all atomic weight of a particular substance corresponds to its molecular weight. In the case of water for example, it is basically composed of 2 Hydrogen atoms and 1 Oxygen atom. Adding the atomic weights of these atoms will result to the molecular weight of H20 or water. The same is true for Carbon Dioxide or C02. Adding the atomic weights of 1 Carbon atom and 2 oxygen atoms will give its total molecular weight. The values given for atomic weight for any substance are considered average and not absolute. This is simply because the atoms may have different isotopes or smaller components resulting to slight variations in the total atomic weights. With variation in atomic weight, the total molecular weight will also vary and so many substances are given their average molecular weight values.
Molecular weight measurement varies depending on the substance being measured. The way liquids are measured for their molecular weights are very much different from the method for measuring the same weight when a solid is involved for example. In gases for example, specific or standard temperature and pressure is also measured when trying to determine the molecular weight of a particular substance. The procedure is also done in accordance with Avogadro’s law involving the number of molecules that is present at specific temperatures and pressures. Other substances like proteins can be measured using the so-called ultra-centrifuge method using the sedimentation rate as the key element.