Finding Molar Mass (Molecular Weight)

What Is Molar Mass?

Molar mass is the mass (in atomic mass units) of one mole of a of a substance. One atomic mass unit (u) is equal to 1/12 the mass of one atom of carbon-12.

It is also sometimes called: Molecular Mass, Molecular Weight, Formula Mass, or Formula Weight.

How can I find the molar mass of an element?

The molar mass of elements is found by looking at the atomic mass of the element on the periodic table. For example, if you want to find the molar mass of carbon, you would find the atomic mass of carbon on the periodic table, and this is equal to the molar mass in grams per mole. So, in our example, carbon has a molar mass of 12.01 grams per mole. There are a few exceptions to this rule. In some cases, the element is usually found in a different form than just one unbonded atom. In the case of hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine, the element is diatomic, meaning that each molecule of the element has two atoms of that element stuck together. As a result, the formula of hydrogen is H2, nitrogen is N2, etc.

This gets weirder for a couple of cases... phosphorus is normally found in clumps of four atoms, P4, and sulfur is found in clumps of eight atoms, or S8.

Still, aside from the exceptions above, all elements have the same molar mass as the atomic masses on the periodic table.

How can I find the molar mass of a chemical compound?

For any chemical compound that's not an element, we need to find the molar mass from the chemical formula. To do this, we need to remember a few rules.

1. Molar masses of chemical compounds are equal to the sums of the molar masses of all the atoms in one molecule of that compound.
If we have a chemical compound like NaCl, the molar mass will be equal to the molar mass of one atom of sodium plus the molar mass of one atom of chlorine. If we write this as a calculation, it looks like this:

(1 atom x 23 grams/mole Na) + (1 atom x 35.5 grams/mole Cl) = 58.5 grams/mole NaCl

2. If you have a subscript in a chemical formula, then you multiply the number of atoms of anything next to that subscript by the number of the subscript.
For most compounds, this is easy. For example, in iron (II) chloride, or FeCl2, you have one atom of iron and two atoms of chlorine. The molar mass will be equal to:

(1 atom x 56 grams/mole Fe) + (2 atoms x 35.5 grams/mole of chlorine) = 127 grams/mole of iron (II) chloride

For other compounds, this might get a little bit more complicated. For example, take the example of zinc nitrate, or Zn(NO3)2. In this compound, we have one atom of zinc, two atoms of nitrogen (one atom inside the brackets multiplied by the subscript two) and six atoms of oxygen (three atoms in the brackets multiplied by the subscript two). The molar mass of zinc nitrate will be equal to (1 atom x 65 grams/mole of zinc) + (two atoms x 14 grams/mole of nitrogen) + (six atoms x 16 grams/mole of oxygen) = 189 grams/mole of zinc nitrate.

For all other compounds, the general idea is the same. Basically, you should know how to find the molar masses of any chemical compound now.

Using a Tool

You can also use our molar mass calculator.