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Molecular Weight

For Chemistry students, the term molecules would be one of the frequently used ones right from the day they start to learn the subject. An important aspect of molecules is the way their weight or mass is calculated in the form of molecular weight, also referred to as molecular mass. The calculation for the same is done by taking the total atomic mass for every individual atom contained, and multiplying it by the number of atoms of the respective element in the molecular formula. In the absence of crystallographic data, modes based on scattering light and viscosity could be used to find the molecular weight, mainly in the case of proteins, as they are large molecules.

Molecular Weight

What Is Molecular Weight?

In simple words, molecular weight is defined as the mass of a molecule or the mass of one mole of a substance.

How to Determine Molecular Weight?

There are various ways in which one can determine the weight of molecules. We will discuss them below.

Mass Spectrometry

In this scenario, there would be the assessment of monoisotopic mass which would be the molecular weight of a tiny molecule, viz., it would contain the most common isotope of every element.  You should notice that there would be some subtle differences from the molecular mass where there would be definitions related to the isotope choices, and hence, the possibility of many a single specific molecular weight arises. The regular periodic table would not have the monoisotopic molecular weights, and to calculate them, they could be traced only on the isotopic masses table. As molecules with a lot of atoms are least likely to contain the highest abundant isotope of every element in an exclusive manner, larger molecules are the ones that mostly rely on average molecular mass. As isotopes are represented within the molecule owing to the statistical distribution of atoms, an average molecular weight (theoretical) could be computed relying on the relative atomic weight on the general periodic table.

Hydrodynamic Method

This method is based on the Mark–Houwink relations, which state that the volumetric proportion of dispersed particles present in a typical solvent is the dependent factor on which the intrinsic viscosity of solutions/suspensions of macromolecules. To be specific, the particular molecular shape would be dependent on the conversion factor of the hydrodynamic size. A range of techniques that cater to the effects of hydrodynamics, like SEC, DLS, DOSY, viscometry and diffusion, would help to describe the molecular weight. The molecular weight could thus be calculated using a range or series of macromolecule-specific standards using the hydrodynamic size.

Static Light Scattering

Traditional methods, like the Zimm method, which relies on light scattering, would be helpful in determining molecular weight. The method employed would either use multi-angle light scattering detectors or classical static light scattering.  The only required external measurement would be the refractive index increment that indicates changes in the refractive index with concentration.

Thus, the steps mentioned above would be more than sufficient to determine the molecular weight in the best possible manner. Students hence could refer to this as a reference tool whenever they wish to find the molecular weight of elements they wish to know more about.

Introduction to Atomic and Molecular Weight

Frequently Asked Questions – FAQs


What is the molecular weight of an element?

The molecular weight of an element is the mass of 1 mole of an element.

How is molecular weight determined?

Molecular weight is determined by several methods. Mass spectrometry, hydrodynamic method, and static light scattering are a few methods to determine molecular weight.

The hydrodynamic method measures the weight of which molecules?

The hydrodynamic method measures the weight of macromolecules.
Test your knowledge on Molecular Weight


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