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What is Molecular Weight?

By Georga
Updated Feb 18, 2024
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Molecular weight is a measurement that represents the mass of a single molecule of some substance. It helps scientists and analysts understand density, volume, and fluid displacement, among other things. Knowing the exact weight of molecules in a certain chemical or compound can also help people gauge how those compounds will react under different conditions. There are a couple of different ways to calculate weight, but tests involving mass spectrometry, fluid displacement, and light refraction tend to be the most common.

Getting a General Average

The easiest way to calculate molecular weight, also sometimes called “molecular mass,” is to add up the total weight of each component. This usually requires prior knowledge of each atom’s basic mass as well as the substance’s molecular composition. Water is a good example. It is made of two hydrogen atoms and one oxygen atoms, and is known chemically as H2O. A researcher would take the weight of hydrogen and multiply it by two, since there are two hydrogen atoms in water, and then add the weight of oxygen. Hydrogen weighs about 1 atomic mass unit (u) and oxygen weighs about 16 u. The calculation therefore is 1 × 2 + 16 = 18 u, so the weight of one molecule of water is approximately 18 u.

Accounting for Isotopes

In most cases molecular mass is a baseline average; the weight of any one molecule may be slightly different than another because of isotopes. Isotopes are variations of elements and have a nucleus 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. Taking account of isotopes will give a more accurate measurement, but usually requires special tools and a more precise process.

Precise Calculation Methods

One of the best ways to get an exact measurement is by using a mass spectrometer, a special machine designed to isolate and identify molecules. The machine produces numbers and representations that analysts must use to piece together a more exact mass reading. These machines are usually housed in labs and academic institutions.

Scientists sometimes also calculate weight using what are known as “hydrodynamic” methods, which basically involve submerging substances in water and studying the amount and extent of displacement. It requires special calibration and a number of tools to both capture and observe dispersed molecules. In some cases researchers can also get a good sense of mass by performing a scattered light analysis, in which beams of light are passed through substances. The Refraction rate, which is basically the rate and speed at which light passes from one side of a substance to the other — or the rate at which it is absorbed, in the case of highly dense substances — can provide the basis for determining molecule-level weight.

Relationship to Molar Mass

Some chemists refer to this measurement interchangeably with molar mass. Molar mass refers to the weight of one mole of a substance, which is similar to atomic mass but is often thought to be more universally precise. 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.

Why it Matters

Knowing how much different molecules weigh is important in a number of different settings, but is usually most critical in laboratory and manufacturing environments. Scientists usually need this information in order to anticipate how different substances will relate to each other, and also to be sure that concentrations and suspensions are stable. Food manufacturers use these sorts of calculations to make sure that different ingredients will blend, for instance, and to get the right taste profile when using sweeteners or emulsifiers with different weights.

Molecular weight is also really important when creating and dosing medication. Pharmaceutical manufactures as well as doctors and nurses often need to know how much a molecule of a given drug weighs in order to know how potent a capsule, injection, or other dose is likely to be. Weight is also really important when it comes to switching medications or varying prescriptions in order to be sure that patients are getting the right concentration.

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Discussion Comments

By anon320311 — On Feb 16, 2013

@adele14: You would probably contain the gas or substance in a closed system and keep the temperature constant. Then apply Boyle's law.

By adele14 — On Apr 10, 2011

How is weight measurable for substances that appear weightless, such as helium gas or cigarette smoke?

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