# How can the periodic table be used to determine molar mass?

Dec 9, 2013

The molar mass of a substance is the substance's mass divided by its amount. The amount of a substance is usually set at 1 mole and it's the substance's mass needs to be calculated to find out the molar mass.

The elements that compose a substance all have an atomic mass. The substance's mass is the sum of all of those atomic masses. The periodic table provides the atomic mass next to or below each element.

For example:
Find the molar mass of ${H}_{2} O$.

The substance, ${H}_{2} O$ or water, is composed of two atoms of hydrogen and one atom of oxygen. To find the molar mass, we need to add the atomic masses of two Hydrogen atoms and one Oxygen atoms.

Equation 1
Molar mass of ${H}_{2} O$ = (2 x atomic mass of Hydrogen) + (atomic mass of Oxygen)

We find the atomic mass of Hydrogen and Oxygen in the periodic table. They are
1.007 g/mole for Hydrogen and 15.999 g/mole for Oxygen.

Plug these values into Equation 1 and you'll find the molar mass of water to be around 18g/mole.