Molecular Weights – Concept of Molecular Mass

Molecular mass is a number equal to the sum of the atomic masses of atoms in the molecule. Molecular mass gives the mass of the molecule relative to the 12-C atom, which is believed to have a mass of 12. Molecular mass is a dimensionless quantity, but it is measured in Daltons, or atomic mass, as a means of expressing mass relative to 1/12 the mass of a single carbon 12 atom. Molecular mass is also called molecular weight. Since the mass is related to carbon 12, it is more appropriate to call this value “relative molecular mass”. A related term is a molar mass, which is the mass of 1 mole of a sample. Molar mass is given in units of grams.

What is Molecular Mass?

Molecular mass is the mass of a molecule. It can be calculated by adding together the atomic masses of the atoms present in the molecule.

For example, H2O is a molecule.

The molecular weight of H2O = 2 × mass of H + mass of O.

The atomic mass of hydrogen is 1 g, and the atomic mass of O is 16.

Therefore, the molecular weight of H2O = 2×1 + 16 = 18g.

Molecular Mass Unit

Usually, the unit for molecular mass is “dalton”. A dalton is exactly 1/12 the mass of a carbon 12 atom when it is not bonded to another atom and its electrons are in the lowest energy configuration. You may also see the term “atomic mass unit” or “AMU,” which can be the same as dalton or defined as 1/16 the atomic mass of oxygen 16. Atomic mass unit is ambiguous whereas dalton is not.

How to Calculate Molecular Mass?

Molecular mass can be calculated by multiplying the atomic mass of each element present by the atomic number of that element in the molecular formula. Then add the atomic number of each element.

E.g. To find the molecular mass of methane, CH4, the first step is to use the periodic table to find the atomic masses of carbon C and hydrogen H:

Atomic mass of carbon = 12.011

Atomic mass of hydrogen = 1.00794

Because there is no subscript after C, you know that there is only one carbon atom in methane. The subscript 4 after the H indicates that there are four hydrogen atoms in the compound. So by adding atomic mass, you get:

Molecular mass of methane = sum of carbon atomic mass + hydrogen atomic mass.

Molecular weight of methane = 12.011 + (1.00794) (4)

Atomic mass of methane = 16.043

The value can be reported as a decimal number or as 16,043 Da or 16,043 amu.

The number of noteworthy digits in the final value. The correct answer uses the least significant number in the atomic mass, which in this case, is the number in the atomic mass of carbon.

The molecular weight of C2H6 is approximately 30 or [(2 x 12) + (6 x 1)]. So the molecule is about 2.5 times heavier than a 12C atom, or about the same mass as a NO atom with a molecular weight of 30 or (14+16).

Molecular Weight Calculation Problem

While it is possible to calculate the molecular weight of small molecules, it is a problem for polymers and macromolecules because they are so large and may not have a uniform formula throughout the volume. For proteins and polymers, experimental methods can be used to obtain the average molecular weight. Techniques used for this include crystallography, static light scattering, and viscosity measurements.

Difference Between Molecular Mass and Molar Mass

1. Unit

We usually use AMU (atomic quality units) or ‘u’ (unified mass) while talking about mass of atoms, molecules and ions.

On the other hand, when we talk about the molar mass, we usually use grams per mole (G).

Note: 1 AMU = 1u = 1.67 * 10-24G (The inverse of Avogadro’s number)

Meaning

The mass of the atom is called atomic mass. It is the mass of one atom of this element (e.g., hydrogen).

For example, the mass of 1 atom of H is 1 AMU.

Similarly, the molecular mass is the quality of this molecule.

The molecular mass of Ca (OH)2 is, for example, 74 AMU.

Molar mass is the mass of the 1 mole of the element.

For example, the molecular mass of hydrogen is 1 g, which means that the mass of 1 mole of hydrogen atom is about 1 g.

Note: In many formulas, you will find the following terms interchange: atomic mass, molecular mass, formula, and molar mass  Remember that molar mass is the atomic mass or molecular mass in grams.

Conclusion

The molecular weight is usually used interchangeably with molecular mass in chemistry, although there is a difference technically in both. The molecular mass is a measure of mass, and the molecular weight is a measure of the force working on the molecular mass. More appropriate terms used in chemistry for molecular weights and molecular mass would be  “relative molecular weight”. Relative molecular weight usually means relative to 1/12 the mass of one atom of carbon-12. Since the mass of one atom of the carbon-12 is defined as 12 dalton (or 12 mass units), “relative molecular weight” refers to the quality given in units or Dalton. Thus, it is mathematically, similar to the molecular mass in grams per mole (by definition, one mole is the number of things that are as many as the number of carbon-12 atoms in 12 grams).