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

where:

is the mass of the substance in grams is the molar mass of the substance is the amount in moles of that substance

References

  1. M. Wieser, Atomic Weights of the Elements 2005 [Online], Available: http://www.iupac.org/publications/pac/2006/pdf/7811x2051.pdf

Authors and Editors

Semaa Amin, Jordan Hanania, Kailyn Stenhouse, Jason Donev