The key difference between molar mass and molecular mass is that the molar mass gives the average mass of the molecule whereas the molecular mass gives the mass of a single molecule.
Atoms can join together in various combinations to form molecules and other compounds. Molecular structures give the exact ratios of atoms; thus, we can write molecular formulas for compounds. These are important in determining the molecular masses or molar masses. We can characterize molecules with their masses. Knowing this is really useful in laboratory work when measuring the compounds for reactions.
What is Molar Mass?
We can call molar mass as molecular weight as well. Thus, it is the mass of a substance for a given amount. The SI unit for the molar mass is g mol-1. This gives the amount of the atoms/molecules/compounds present in one mole of the substance. In other words, it is the mass of Avogadro number of atoms/molecules or compounds.
It is important to measure the weight of atoms and molecules in the practical scenario. But it is difficult to weigh them as individual particles, since their masses are really small according to the normal weighing parameters (such as grams or kilograms). Therefore, to fulfill this gap and to measure the particles in a macroscopic level molar mass concept is useful.
The definition of molar mass is directly related to the carbon-12 isotope. The mass of one mole of carbon-12 atoms is exactly 12 grams, which is its molar mass, it is exactly 12 grams per mole. We can calculate the molar mass of molecules containing the same atom like O2 or N2 by multiplying the number of atoms by the molar mass of the atoms. Moreover, we can calculate molar mass of the compounds like NaCl or CuSO4 by adding the atomic masses of each atom.
What is Molecular Mass?
Molecular mass of a compound is the mass of one molecule. We can measure this using atomic mass units (amu). 1 atomic mass unit is the one-twelfth of the mass of a C-12 isotope. When we divide the mass of a molecule by the mass of one-twelfth of the mass of a C-12 isotope, we get the relative mass. Furthermore, molecular mass differs according to the isotopes that the molecule contains. When calculating the molecular mass, we should consider the relevant isotopic mass.
What is the Difference Between Molar Mass and Molecular Mass?
Molecular mass gives the mass of a single molecule (sum of the atomic masses in the molecule), whereas molar mass gives the average mass of the molecule (mass of Avogadro number of molecules). Hence, this is the key difference between molar mass and molecular mass. Therefore, the molecular mass of a molecule containing heavier isotopes can be few units higher than the molar mass. Otherwise, the value of the molar mass and molecular mass is quite the same for a similar molecule; only the units are different. One significant difference between molar mass and molecular mass is that the unit of the molecular mass is amu, and the unit of the molar mass is gmol-1.
Furthermore, it is accurate to use the molecular mass in calculations of higher amounts of molecules than using the molar mass. Moreover, in measurement, one difference between molar mass and molecular mass is that the molar mass is a measurement, which we can give for an atom, molecule or compound. But we can determine the molecular mass only in the molecules. If it is the mass of an atom, we use different terminology; atomic mass.
The below infographic presents the difference between molar mass and molecular mass in tabular form.
Summary – Molar Mass vs Molecular Mass
The value of the molar mass and molecular mass is quite the same for a similar molecule. But, for a molecule containing heavier isotopes, the molecular mass is higher than the molar mass. In summary, the key difference between molar mass and molecular mass is that the molar mass gives the average mass of the molecule whereas the molecular mass gives the mass of a single molecule.
1. “Molecular Mass.” Wikipedia, Wikimedia Foundation, 26 Sept. 2018. Available here