Molar Mass and How to Find It

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How to Find Molar Mass
Molar mass is the mass in grams per mole of a substance. Find it by adding up the element atomic masses.

In chemistry, the molar mass is the mass in grams per mole (g/mol) or kilograms per mole (kg/mol) of a substance. Molar mass is an intensive property of matter, meaning its value does not depend on sample size.

How to Find Molar Mass

Follow these simple steps to find the molar mass of a compound:

  • Start with the chemical formula.
  • Using a periodic table, look up the atomic mass of each element in the formula. (Note: Use different values if you are working with a known isotope.)
  • Add up the atomic mass values of each element, according to the chemical formula. For each element, multiply the atomic mass by the subscript following its symbol. If there is no subscript, it’s the same as multiplying by ‘1’.

Example #1: Find the Molar Mass of an Element

For example, find the mass of one mole of sodium. Do this by looking up sodium (Na) on the periodic table. The relative atomic mass is the same as the molar mass (except molar mass is in g/mol). The molar mass of sodium is 22.99 g/mol. Now, you know the atomic number of sodium is 11, so you may wonder why the molar mass is not exactly 22 (11 protons and 11 neutrons). This is because the average isotope abundance of sodium in the Earth’s crust includes other isotopes besides sodium-22. Remember, molar mass is an average mass per mole.

As another example, find the mass of one mole of oxygen gas. Oxygen gas is a diatomic molecule (O2). First, look up the atomic mass (atomic weight) of the element, which is 16.00. Next, multiply this value by 2 (the subscript following the symbol for oxygen, O). The molar mass of O2 is 16.00 x 2 = 32.00 g/mol.

Example #2: Find the Molar Mass of NaCl

Apply the same step and find the molar mass of table salt or NaCl.

  1. The formula is NaCl.
  2. Atomic mass of Na = 22.99; atomic mass of Cl = 35.45.
  3. Molar mass of NaCl = 22.99 + 35.45 g/mol.

Example #3: Find the Molar Mass of CO2

Find the molar mass of carbon dioxide:

  1. The formula is CO2.
  2. Atomic mass of C = 12.01; atomic mass of O = 16.00.
  3. Molar mass of CO2 = 12.01 + (16.00 x 2) = 44.01g/mol

Molar Mass vs Molecular Mass

Most of the time, people use the terms “molar mass” and “molecular mass” interchangeably. But, they are not exactly the same as each other.

First, molecular mass is either unitless or else reported in daltons (Da) or atomic mass units (amu or u). On the other hand, the unit for molar mass is grams per mole (g/mol) or kilograms per mole (kg/mol).

Second, molecular mass describes the mass of a single molecule or type of molecule. Meanwhile, molar mass is a bulk property, reflecting the average mass of particles in a material. In other words, the average chemical formula and isotope ratio of elements matter.

So, the molar mass of NaCl presumably is different in a sample from Earth versus one collected on Venus because of slight variations in isotope abundance of the elements. A more common scenario is calculating the molar mass of a sample of a polymer that contains differing numbers of monomer subunits. Another use of molar mass is finding average mass per mole of DNA or RNA, which contain varying numbers of different nucleotides. Molar mass finds value in mixtures, where molecular mass isn’t applicable.

References

  • International Union of Pure and Applied Chemistry (1993). Quantities, Units and Symbols in Physical Chemistry (2nd ed.). Oxford: Blackwell Science. ISBN 0-632-03583-8.
  • IUPAC (1997). “Relative molar mass”. Compendium of Chemical Terminology (the “Gold Book”) (2nd ed.). Oxford: Blackwell Scientific Publications. doi:10.1351/goldbook.R05270
  • International Bureau of Weights and Measures (2006). The International System of Units (SI) (8th ed.). ISBN 92-822-2213-6.
  • Possolo, Antonio; van der Veen, Adriaan M. H.; Meija, Juris; Hibbert, D. Brynn (2018). “Interpreting and propagating the uncertainty of the standard atomic weights (IUPAC Technical Report)”. Pure and Applied Chemistry. 90 (2): 395–424. doi:10.1515/pac-2016-0402