Molar mass

Molar mass (M) is a physical property, defined as the mass of a given element or molecule per mole of that substance. Since a mole is defined as the amount of a substance and substances have different masses, each element or molecule will have a different molar mass. For example, if someone has a dozen bricks and another person has a dozen feathers, they have the same amount of their substance, but a vastly different total mass. However when talking about moles, instead of only having a dozen, there is in fact 6.022141 x 1023 of a given substance in a mole (known as Avogadro's number). This comparison is seen below:

1 dozen = 12 objects
1 mol = 6.022141 x 1023 objects

In SI units, the molar mass is given by kg/mol, however by convention the molar mass is expressed in units of g/mol. The molar mass of any element or molecule is given by the sum of the atomic weights multiplied by 1 g/mol. A list of some molar masses can be seen below.[1]

Element/Molecule Molar Mass (M)
Hydrogen (H) 1.00794 g/mol
Carbon (C) 12.0107 g/mol
Iron (Fe) 55.845 g/mol
Uranium 238.02891 g/mol
Water (H2O) 18.01528 g/mol
Carbon dioxide (CO2) 44.0095 g/mol

By knowing the molar mass of a substance, having any mass of that substance allows one to know how many moles of it they have. This is given by the equation



[math]m[/math] is the mass of the substance in grams
[math]M[/math] is the molar mass of the substance
[math]n[/math] is the amount in moles of that substance

For Further Reading


  1. M. Wieser, Atomic Weights of the Elements 2005 [Online], Available: