How to Find Molar Mass of a Compound

What is molar mass?

Molar mass is the weight, in grams, of one mole of any element or compound. Molar mass is useful all over chemistry, so being able to find molar mass is an ultra important skill.

If you've made it here, chances are that you're just learning how to calculate molar mass for the first time or are brushing up on it. Don't worry if you don't get it right away, with lots of practice and patience, you'll get a good handle on it.

Let's get started by taking a look at the periodic table and going over some easier examples. Later, we'll go in depth with some tougher ones, but I'll quiz you along the way. So let's get our hands dirty.


Using the Periodic Table to Find Molar Mass

You'll need a periodic table to figure out molar mass. If you're in a bind, you can print one out or visit Ptable. Ptable has an amazing periodic table that's a lot of fun to play around with.

Once you've got access to a periodic table, you'll want to get to know what its atomic mass is.

To the right, copper is shown. Copper has an atomic number of 29, a symbol of Cu, and an atomic mass of 63.55. We'll focus primarily on the atomic mass and symbol for this tutorial, so you can ignore the atomic number for now.

The 63.55 means that copper weighs 63.55 grams per mole.

Quick Quiz: Molar Mass of Diatomic Compounds

Molar mass of a diatomic element

The above question asks you to find out how many grams are in two moles of copper.

Let's go through this concept, this time with oxygen. Oxygen is a diatomic element which means it usually exists in the form of O2.

There are seven diatomic elements. The other six are hydrogen (H2), nitrogen (N2), fluorine (F2), chlorine (Cl2), iodine (I2), and bromine (Br2.)

Fluorine weighs 18.998 grams per mole. The F2 molecule weighs twice as much. This is because there are two fluorine atoms in each molecule.

How to calculate how many grams are in two moles of Fluorine.
How to calculate how many grams are in two moles of Fluorine. | Source
Potassium chloride
Potassium chloride | Source

Molar Mass of a Compound

Calculating the molar mass of a compound can be a little bit trickier. This is because you're dealing with multiple elements (each with differing molar masses.)

The formula for potassium chloride, pictured above, is KCl. Essentially you have one mole of K (with a molar mass of 39.098 grams) and one mole of Cl (with a molar mass of 35.45 grams) added together to create one mole of KCl. This can be expressed mathematically as in the image below.

How to calculate how many grams are in a mole of KCl.
How to calculate how many grams are in a mole of KCl. | Source

Calculating the Molar Mass of Larger Compounds

If you have a larger compound, particularly one with a subscript, you'll need to multiply the molar mass of the the element by the subscript.

Let's try this with a larger compound: Na3PO4

Sodium (Na) has a subscript of three, phosphorus (P) has no subscript, and oxygen (O) has a subscript of four.

To express this mathematically:

How to calculate the molar mass of Na3PO4.
How to calculate the molar mass of Na3PO4.

Finding the Molar Mass of Compounds with Parentheses

Compounds with parentheses can look confusing, but it just means that you have to multiply the molar masses of the compounds within the parenthesis by the subscript outside the parenthesis.

Let's consider Ca(OH)2. This means we have one calcium (Ca) and two OH groups. (That's two oxygens and two hydrogens.)

To find the molar mass of Ca(OH)2:

How to find the molar mass of Ca(OH)2
How to find the molar mass of Ca(OH)2 | Source

Quick Quiz: Molar Mass of Compounds

© 2014 Melanie Shebel

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1 comment

David Warren profile image

David Warren 2 years ago from Nevada

Thank you for a great read, and loved the quiz! Highly unlikely I'll need to calculate molar mass but I love to read and learn everything, well as much as possible anyway. One of the fun parts of getting older, I could come back in twenty-four months and would have to read the article yet again to complete the quiz lol. KEEP WRITING, voted up!

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