Molar mass

We can equate the numerical quantity of mass of a single ATOM (in amu) to the mass of a mole of the same atoms (in grams). If we are talking about compounds, which are made up of two or more atoms, we can equate the numerical quantity of mass of a single MOLECULE to the mass of a mole of the same molecules (in grams).

For example,

1 water molecule

H2O has a mass of 18.015 amu.
2 H atoms:	of 2 x 1.008 amu = 2.016 amu
1 O atom:	1 x 15.999 amu = 15.999 amu
Total mass = 18.015 amu

1 mole of water molecules

H2O has a mass of 18.015 grams.
2 moles of H atoms:	2 x 1.008 grams = 2.016 grams
1 mole of O atoms:	1 x 15.999 grams = 15.999 grams
Total mass = 18.015 grams

1 mole, or 6.022x 10²³, of water molecules has a mass of 18.015 grams. This is the molar mass of water.

Example

Calculate the molar mass of Na₂CO₃

Types of Atoms	Number of atoms	Atomic mass (g/mole)	Mass of each type of atoms (g/mole)
Na	2	22.990	45.98
C	1	12.011	12.011
O	3	15.999	47.997
Molar Mass of Na2CO3			105.988 g/mole

Note: The mass of 1 formula unit of Na₂CO₃ is 105.988 amu.

Summary

The unit of molar mass

Content suitability

BCIT courses: CHEM 0011