Molecular Weight

A molecule’s weight is the sum of the weights of the atoms that make it up. The unit of weight is the dalton, which is one-twelfth the weight of a 12C atom. Water’s molecular weight (MW) is, thus, 18 daltons. Because different isotopes of an element are present in different molecules of the same compound, their molecular masses can differ. The following sections will give you the meaning of molecular weight and its formula.

What is Molecular Weight?

The sum of the atomic weight values of the atoms in a molecule is expressed as molecular weight. Molecular weight is often used in chemistry to evaluate chemical reactions and equations’ stoichiometry. M.W. or MW are two abbreviations for molecular weight and can be expressed in terms of atomic mass units (amu) or Daltons (Da), or it can be unitless.

Molecular mass and relative molecular mass are not the same as molar mass. But these concepts are related. The molar mass is expressed as the mass of a given element divided by the amount of that element. It is expressed in grams per mole (g/mol). The molar mass is commonly the more appropriate figure when considering a substance’s macroscopic (weighable) quantities.

Why is it Necessary to Know a Compound’s Molecular Weight?

Molar mass can also analyse experiment results. When two equal amounts of molecules of different compounds take up different volumes, it implies that the molecules of the greater volume are larger than the molecules of the smaller volume. 

Assume you want to examine the reactions of honeybees to different types of sugar solutions. Making various solutions and seeing which one the bees choose to extract is one method of achieving this. 

We can give the bees two choices: a 35% solution of sucrose (common table sugar) and a 35% solution of glucose (a natural component of honey). This would necessarily involve dissolving 350 parts by weight (e.g. grams) of sugar in 650 parts (g) of water in each case, which would produce 1000 g of each solution. 

However, there is a flaw in this plan. The honeybee’s ability to react to the presence of sugar mixed with water is proportional to the number of sugar molecules in a given volume of solution. 

The weight of a sucrose molecule (MW = 342) is about double that of a glucose molecule (MW = 180). As a result, a 35% glucose solution contains nearly twice as many molecules as a 35% solution of sucrose.  To solve the problem, come up with solutions with the weights of sucrose and glucose in the ratio 342:180. Then, each solution will have the same concentration of molecules i.e. each solution would contain an equal number of molecules drop for drop.

Molecular Weight Formula

The molecular weight of a compound is calculated using its molecular formula. The atomic weight of each type of atom is multiplied by its atomic weight, and the weight of other atoms is added.

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

Hexane, for instance, has the chemical formula C6H14. The subscripts here refer to the number of each atom type, so each hexane molecule contains 6 carbon atoms and 14 hydrogen atoms. 

We can use the Periodic Table to determine the atomic weights of carbon and hydrogen.

Atomic weight of Carbon = 12.01 and Atomic weight of Hydrogen = 1.01

Molecular Weight Formula = (number of carbon atoms)(C atomic weight) + (number of H atoms)(H atomic weight) 

⇒ Molecular weight of hexane = (12.01)(6) + (14)(1.01)

= 72.06 + 14.14

= 86.20 amu

Conclusion

Molecular weight is the mass of a substance’s molecule. It is calculated using 12 as the atomic weight of carbon-12. In practice, it is calculated by adding the atomic weights of the atoms that make up the molecular formula of the substance. A hydrogen molecule’s molecular weight (chemical formula H2) is 2.02. But it can be in millions for many complex organic molecules (e.g., proteins, polymers).

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