Molar Mass - Energy Education

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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

n=mM

where:

m is the mass of the substance in grams M is the molar mass of the substance n is the amount in moles of that substance

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References

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

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