How To Determine Molecular Weight?

The term molecular weight means the calculated sum of the total atomic weight of the given atoms in a molecule. This term is mostly used to determine stoichiometry in the given equation. The most common term by which molecular weight is abbreviated is MW. The molecular weight is either written in the form of atomic mass units (amu) and Daltons (Da) or unitless. The molecular weight formula is used to determine the weight in atomic mass units. Molecular weight and atomic weight or described in relation to the mass of an isotope carbon 12 that has been given a value of 12 amu. Due to carbon being a mixture of isotopes of carbon, its atomic weight is not precisely 12.

Calculation Of Molecular Weight

Calculation of the molecular weight formula is based on many other small units. To know how to calculate molecular weight, we first need to know the smaller units—just based on the molecular formula of the given compound. The total count of each atom is then multiplied by its atomic weight, which is later added to the given words of the other atoms. The atomic weight of all elements can be found on a periodic table. The molecular weight formula is:

Molecular weight = (the number of element A atoms) (atomic weight of element A) + (number of element B atoms) (atomic weight of element B)

The molecular weight formula of any compound can be measured as. For example, the molecular weight of water,

How To Calculate Molecular Weight Of Water?

This is the formula where there are two hydrogen atoms and one atom of oxygen. The weight of each atom of hydrogen is 1, and the weight of one atom of oxygen is 16. First, multiply the number of atoms of hydrogen by its atomic weight
=> 2 x 1 = 2
Next, multiply the number of atoms of oxygen by its atomic weight
=> 1 x 16 = 16
Now add both these
=> 2+ 16 = 18
Hence, the answer is 18 amu. This is how you calculate the molecular weight of water.

Let us take another example to calculate the molecular weight of acetic acid, i.e., CH3COOH.

This is the formula where there are two carbon atoms, four hydrogen atoms and two oxygen atoms. The weight of each atom of carbon is 12, hydrogen is 1, and the weight of one atom of oxygen is 16. First, multiply the no. of atoms of carbon by its atomic weight
=> 2 x 12 = 24
next, multiply the number of atoms of hydrogen by its atomic weight
=> 4 x 1 = 4
next, multiply the number of atoms of oxygen by their atomic weight
=> 2 x 16 = 32
Now add both these
=> 24 + 4 + 32 = 60
Hence, the answer is 60 amu.

The determination of molecular weight can be determined. To do so, first, you need to understand a few things. The mansion data on the molecular weight of a certain compound directly depends on the size of that particular molecule. Mass Spectrum analysis is one of the most used methods to determine the molecular mass of molecules sized from small to medium. 

Difference between molecular weight and molecular mass

Molecular weight is very often confused with the term molecular mass. There is a technical difference between molecular weight and molecular mass. The term molecular mass is used to determine the mass of a certain compound or element, whereas molecular weight is a term that is used when measuring the force that is acting upon the molecular mass of a certain compound or element.

Relative molecular mass can be a more fitting term for both molecular mass and molecular weight altogether in chemistry. The unit used to determine molar mass is kg/mol, the term used to measure molecular weight in atomic mass units. Both molecular weight and molecular mass have different formulas for their calculation that are used to measure molecular mass.

The molecular weight formula of a substance is:

1/12 (mass of one molecule of a substance / divided by the mass of one carbon – 12 atom )

and the formula for molecular mass of a substance is:

molar mass = mass of substance (kg) / amount of substance (mol)

Conclusion

From this article, we got to know what molecular weight is. We also learned the molecular weight formula and got to know how to use it. Knowing about the difference between molecular weight and molecular mass also comes in handy since these two terms are often confused. Methods like viscosity and light scattering are used to measure the weight of larger molecules like proteins and DNA. Other methods like Viscometry, size exclusion chromatography, diffusion ordered nuclear magnetic resonance spectroscopy, and Zimm’s method of light scattering is also used to measure the molecular weight.