Molar Mass

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By Caroline Monahan

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What is the molar mass of a molecule?

The molar mass tells you the mass of a compound/molecule per unit mole.
Molar mass is calculated the same way as molecular mass.
Molar mass and molecular mass are almost the same. But, the molar mass has units of g/mol. The molecular mass has units of amu or daltons (Da).

Molar Mass unit: g/mol

How do you calculate the molar mass of a compound?

You simply add together the atomic weight of each element in a molecule.

Remember, atomic mass is the sum of the protons and neutrons. One AMU is defined as $\frac{1}{12}$ the mass of carbon-12. But, the atomic weight averages the relative abundance of the isotopes. Sometimes these terms get used interchangeably.

Reminder on where to find the atomic mass of an element:

Image showing where to find the atomic mass of an element (the number on the bottom of the square, in this case, the atomic mass of carbon is 12.011) — Image source: By Caroline Monahan

Example: The molecular formula for the organic compound propanone is $C_{3}H_{6}O$ . Find its molar mass.

Remember, the numbers after the carbon (3) and hydrogen (6) tell you how many are in the molecule. So there are 3 carbons and 6 hydrogen atoms in this molecule. The oxygen doesn't have a subscript because there is only one.

Carbon: 3 X 12.011 = 36.033

Hydrogen: 6 X 1.008 = 6.048

Oxygen: 1 X 15.999 = 15.999

Add the masses together:

36.033 + 6.048 + 15.999 = 58.08 g/mol ← This is the molar mass of $C_{3}H_{6}O$

How do you determine the number of moles of an element in a certain molecule?

Image showing how to convert between moles and mass. To convert from mass to moles multiply by mols/g and to convert from moles to mass multiply by g/mol — Image source: By Caroline Monahan

Using these conversion factors, you can determine the number of moles or mass of a compound!

Example: A student measures out 14.66 grams of sodium bicarbonate or baking soda, $NaHCO_{3}$ . How many moles of baking soda does she have?

First determine the molar mass of the baking soda:

Sodium: 1 X 22.990 = 22.990

Hydrogen: 1 X 1.008 = 1.008

Carbon: 1 X 12.011 = 12.011

Oxygen: 3 X 15.999 = 47.997

22.990 + 1.008 + 12.011 + 47.997 = 83.92 g/mol $NaHOC_{3}$

Use the correct conversion factor to determine the number of moles!

$14.66g\cdot\frac{1mol}{83.92g}=0.1747mols$ $NaHOC_{3}$

The grams cancel out, leaving us with just the amount of mols of $NaHCO_{3}$

Now try to convert from moles to mass!

A student has 0.5677mols of hydrogen peroxide ( $H_{2}O_{2}$ ). What is the mass in grams of hydrogen peroxide present?

Video

By Eric Sears

(Video) How To Calculate The Molar Mass of a Compound - Quick & Easy!

by The Organic Chemistry Tutor

In this video, The Organic Chemistry Tutor demonstrates molar mass calculations. He starts by showing a couple of pure elements (nitrogen and fluorine). You'll learn that those are easy. You only have to look at the periodic table. Next, he examines larger molecules that consist of one element (ozone and octasulfur). But, the difficulty increases quickly. Soon you'll encounter a couple of covalent compounds (carbon dioxide and silicon tetrafluoride). Finally, he calculates some ionic compounds with polyatomic ions (calcium phosphate and vanadium hydrogen phosphate). Those are massive compounds! If you want to understand how to calculate the molar mass, this video is for you.