Difference Between Similar Terms and Objects

Difference Between Molar Mass and Atomic Mass

Molar Mass vs Atomic Mass

During our high school chemistry classes, as well as my general chemistry subject during college, I can still vividly remember that we were to familiarize ourselves with the table of elements. Though we were not required to memorize the whole table, but we were required to familiarize ourselves with the chemical properties, what is the lightest element, what is the heaviest element, what are the different families of the element, and so on and so forth. To be honest, most people do not like chemistry because it’s just like math in which people have numerous computations to perform.

One of the topics that we tackled in the table of elements is the atomic mass and molar mass. What could be the differences between them?

First, molar mass is defined as the mass of a mole or one mole. It can also be identified with single elements, single molecules, and other substances. Molar mass is the same as molecular weight. However, the main difference is that molecular weight only describes the weight of the molecules over the elements. Both the molar mass and molecular weight are expressed in units of grams per mole.
For example, let us compute for the molar mass/molecular weight of the compound water. Water is also known as H2O. It is composed of two hydrogen atoms and one oxygen atom. An oxygen atom weighs 16 grams per mole. A hydrogen atom weighs 1 gram per mole. So multiplying 2 hydrogen atoms would yield 2 grams per mole of hydrogen plus 16 grams per mole of oxygen equals 18 grams per mole. Thus, the molecular weight or molar mass of water is 18 grams per mole.

Atomic mass, on the other hand, is the mass of the atom not in motion but at rest. Atomic mass is also the number of protons and the number of neutrons. The atomic mass of an atom is described as the sole weight of an isotope and not the average weight. Atomic mass is measured via mass spectrometry. Molar mass is computed from the atomic weight. There are no units of measurement for atomic masses because these are relative masses meaning these are “unitless.”


1.Molar mass is the mass of one mole per single element while atomic mass is the mass of an atom at rest or is the number of protons and neutrons.
2.Molar mass is measured in grams per mole while atomic mass is “unitless.”
3.Atomic mass is measured via mass spectrometry while molar mass is computed via atomic weight.

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  1. I beg to disagree. Atomic mass of an element is the weighted average mass of its isotopes. It is basically the same as the mass number (the number of protons + number of neutrons in an element). It is also similar to molar mass (mass of one mole of the substance). Molecular Weight, on the other hand, is not similar to atomic mass, neither it is similar to mass number nor molar mass. Molecular weight is the mass of the substance in amu that corresponds to the chemical formula of the substance. And to calculate molar mass, you need to use the atomic masses of all the elements that make up the substance(molecule).

    These definitions are taken from Chemistry Reader 2A by Andreas Toupadakis.

    • My bad. I meant to say molar mass is the same as molecular mass. However, it is different from atomic mass and mass number.

  2. One thing ,in case of monoatomic metal 1 molecule will be equal to 1 atom of the element.for ex sodium a molecular mass =23g and atomic mass is 23amu
    So ,when to consider atomic mass or molecular mass in chemical reaction

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