Calculating the Molar Mass of a Substance

The mass of a mole of substance is called the molar mass. To calculate the molar mass of a substance you need a periodic table and the chemical formula.

Finding the Molar Mass of an Element

If you’re trying to find the molar mass of an element, then you’ll take the average atomic mass listed in the periodic table.

If you’re looking at the element of copper you would look in the periodic table and find that it has an average atomic mass of 63.55 AMU, so that’s the molar mass.

Now we’ll write it with a different unit because it’s also our molar mass, and that will be grams over moles: \(\frac{63.55 \text{ g}}{\text{mol}}\). That’s the molar mass of copper.

That’s how you find the molar mass of an element.

Finding the Molar Mass of a Compound

Now the molar mass of a compound can be calculated by summing the molar masses of each atom and the chemical formula. You kind of follow the same protocol for finding the molar mass of an element, but then you combine all those molar masses together to find the molar mass of the compound.

I’m going to show you a 3-step process for finding the molar mass of a compound.

First, find the atomic mass of an atom. You’re going to start with one atom at a time. After you’ve found the atomic mass, multiply the atomic mass by the subscript. That way, you’re finding the total molar mass for all of that atom because you’re finding the atomic mass, then you’re multiplying it by however many times it appears in the compound. By multiplying the atomic mass by the subscript you’re getting the product, you’re getting the molar mass product for that atom.

Then, to find the molar mass of the compound, you’re going to add all those products together. If you were trying to find the molar mass of the compound carbon dioxide, which looks like this—CO2—and the molar mass is 44. Then the way you would find that is first you would find the atomic mass of carbon, which is 12, and then you’d multiply it by the subscript, which is just an understood 1 right there, so the product there would be 12. Then you’d do the same thing with oxygen. You’d find that its atomic mass is 16. You would multiply it by its subscript of 2, which is 32.

Then finally you add all products, so 12 plus 32 is 44. That’s the total molar mass of the compound CO2. Again, the mass of a mole substance is called the molar mass, and to find the molar mass of a substance you just need a periodic table and the chemical formula.