Atomic Mass vs Molar Mass: The Basic Concepts of Chemistry

Atomic mass, also known as atomic weight is the mass of an individual atom expressed in atomic mass units (a.m.u). One a.m.u is the mass of an element relative to the 1/12 mass of a carbon atom. On the other hand, the molar mass is the mass of the one mole of particles ( atoms or molecules) expressed in grams per mole (g/mol).

Atomic mass is so small that it cannot be measured individually. That is why scientists have to use relative terms to define the mass of atoms. For example, the mass of one hydrogen atom (protium) could not be measured directly.

The atomic mass unit (a.m.u)

If we assume for an instant that C-12 is 12 a.m.u with 6 protons and 6 neutrons, and as a hydrogen atom (with one proton) is 12 times lighter than the C-12 atom, the relation between them is 1/12. The value of 1 a.m.u is 1.66 x 10-24 g which is exactly the mass of 1 mole of carbon atoms.

Molar mass unit (g/mol)

Molar mass is a macroscopic (large scale) unit of masses of atoms or molecules. Molar mass is the weight of the Avogadro number of particles of a substance which is why its unit becomes, the number of grams of a substance in one mole of a substance (g/mol).

NA = 6.022 x 1023

The detailed differences between atomic mass and molar mass are as below.

Atomic Mass Molar Mass
Atomic mass is the mass of an individual atom Molar mass is the mass of one mole of particles (atoms and molecules)
It is expressed in a.m.u also known as daltons (Da) or u It is expressed in grams per mole abbreviated as g/mol
Atomic mass sums up the weight of total protons and neutrons in a nucleus
Molar mass is the weight of Avogadro number (6.022 x 1023) of particles
Atomic mass can also be termed atomic weight, although mass is more preferable Molar mass is also known as molecular weight
It is used for atomic (micro) level It is used at the macroscopic level
Average atomic masses include masses of isotopes as well. Relative atomic masses however, are different for isotopes Molar masses, being large scale atomic masses include isotopic masses as well
Examples of atomic masses are; Na=22.98977amu, Cl=35.453amu, O=15.9994amu Examples of molar masses are; Na=22.98977g/mol, Cl=35.453g/mol, O=15.9994g/mol

Besides these differences, a similarity exists between atomic mass and molar mass, i.e. they always have the same values.

All masses are derived relative to 12C

An atom is very small, in the order of picometers. Its mass cannot be measured directly by any known technique. It has to be related to some known mass which can be kept as a standard. Now, as there are isotopes in compounds as well, the standard has to be the one with the highest relative isotopic abundance, otherwise, the answers will not be as certain. This standard is met by element carbon, which has an isotopic abundance of almost 98.89% for carbon-12 (12C).

Atomic Mass

The mass of an atom can be found no matter how small it is unless the number of protons and neutrons in its nucleus is known. This is accomplished via spectrometric techniques. The mass number (number of protons and neutrons) of a sample atom when related to the standard 12C, (1/12th of the mass) gives the mass of an atom, which we call atomic mass or weight.

The mass of individual atoms is very small and cannot be expressed in terms of grams or kilograms. For this reason, chemists use the relative atomic mass scale instead of atomic masses. The relative mass of an atom is called atomic mass or atomic weight.

Unit of Atomic Mass (AMU)

One atomic mass unit is equal to 1/12 of the mass of the carbon-12 atom.

The atomic masses are not the same as the atomic numbers in the periodic table. This is because most elements occur in nature as a mixture of isotopes. Therefore, the average atomic masses of atoms are is taken as the average of the masses of radioactive isotopes. This is why atomic masses usually appear in decimals. For example, the atomic mass of C is 12.011. It can be calculated, as explained below.

Calculating the average atomic Mass

The first step in average atomic mass determination is the correct determination of the number of isotopes and their relative abundances of these isotopes. Such information is obtained by spectrometric techniques (mass spectrometer).

For example, Hydrogen has three isotopes (1H1 ≈ 99.972%, 1H2 ≈ 0.0156%, 1H3 ≈ 10-18%), Chlorine has two isotopes (Cl35= 75%, Cl37= 25%), etc.

The atomic mass of a given element can be determined by obtaining the sum of the product of the masses of isotopes and their percentage abundances.

Examples

1. Carbon atoms contain a mixture of 98.89% of 12C isotope with a mass of 12.00000 a.m.u, and 1.11% 13C isotope with a mass of 13.00335 a.m.u. The average atomic mass of carbon will be as;

Average atomic mass = Σ (mass of individual isotope) (Its percentage abundance)

(12.00000 amu) (98.89 / 100) + (13.000335 amu) (1.11 / 100)

C = 12.011 a.m.u

2. Lithium atom contains a mixture of 7.5% of 6Li and 92.5% of 7Li with 6.01 a.m.u and 7.02 a.m.u masses. The average atomic mass of lithium will be as;

Average atomic mass = Σ (mass of individual isotope) (Its percentage abundance)

(6.01 amu) (7.5 / 100) + (7.02 amu) (92.5 / 100)

Li = 6.94 a.m.u

Molar Mass

Molar mass is the mass in grams of one mole of a substance. Its numerical value is equal to the formula mass that is expressed in a.m.u. For example, glucose has a formula mass of 180.0 a.m.u. So, the molar mass (mass in gram of one mole) of glucose is 180.0 g/mol.

There must be confusion here about molar mass being the same as molecular mass. So, to be very clear, they are not the same.

The formula mass of a compound is the sum of the atomic masses of atoms in a formula unit of that compound, while the molecular mass of the compound is the sum of the atomic masses of atoms in one molecule of the compound.

Below are some examples of calculating molar masses of compounds.

Calculating Molar Masses

1. The molar mass of NaCl

1 atom of Na = 22.9897 g/mol

1 atom of Cl = 35.453 g/mol

Molar mass = Σ (molar masses of individual atoms)

Molar mass of NaCl = 22.9897g/mol + 35.453 g/mol

NaCl = 58.4427 g/mol

2. The molar mass of O2

1 atom of O2 = 15.999 g/mol

Molar mass = Σ (molar masses of individual atoms)

Molar mass of O2 = 15.999 g/mol + 15.999 g/mol

O2 = 31.998 g/mol

3. The molar mass of CO2

1 atom of C = 12.011 g/mol

1 atom of O = 15.999 g/mol

Molar mass = Σ (molar masses of individual atoms)

Molar mass of CO2 = 12.011 g/mol + 15.999 g/mol + 15.999 g/mol

CO2 = 44.009 g/mol

Key Takeaway(s)

  • The molar mass and Atomic mass are different quantities with different units, yet they have the same values.
  • Similarly, molecular weights and molar masses are different quantities with the same values.

Atomic Mass vs Molar Mass The Basic Concepts of Chemistry

Concepts Berg

What Is the difference between molar mass and molecular weight?

Molar mass is the mass of mole in a substance while molecular weight tells the mass of a molecule in a particular substance.

How do you convert molar mass to atomic mass?

Atomic and molar masses are interconvertible because they both are the weights of the same element. Moreover, a.m.u is numerically equal to the grams in one mole of a substance.

What Is the difference between a molecule and a mole?

The molecule is the combination of atoms while a mole is equal to Avogadro’s number of particles, which is 6.022 x 1023 particles.

Are molar mass and molecular mass the same thing?

No, they are different. Molar mass is expressed in g/mole while molecular mass is the mass of a molecule of a compound in a.m.u.

What is the relationship between molecular weight and vapor pressure?

Compounds with high molecular weights have lower vapor pressure. This is because of their increased number of electrons, resulting in more intermolecular forces.

How does molar mass affect the rate of diffusion?

The rate of diffusion is inversely proportional to the square root of its molar mass. By Graham’s law of diffusion;

r ∝ 1/√d

Is molecular weight the same as molecular mass?

Mass and weight are interchangeable quantities in terms of atomic level.

What is the molar mass of ice?

The molar mass of water is exactly 18.01528 g/mol.

Reference books

  • Chemistry in Quantitative Language: Fundamentals of General Chemistry, by Christopher O. Oriakhi (MIT, Nike, hp, RSC, etc)

Reference links

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