In chemistry, one of the hardest concepts for students to grasp is the mole.
Not the mole that burrows underground, and not the mole that you may have on your skin. I’m talking about the chemistry mole.
What is a Chemistry Mole?
A mole is a word that stands for a number, like the words pair, dozen, and gross.
When someone has a pair of shoes, we know they have two shoes. Pair means two.
When someone has a dozen eggs, we know they have 12 eggs. Dozen means twelve.
Though not as widely known, a gross is a dozen dozens. If someone has a gross of pencils, they have a dozen dozens, or 144 pencils.
If someone has a mole of atoms, they have 602,000,000,000,000,000,000,000 atoms.
Wondering how to pronounce that number? It’s 602 thousand billion billion or 602 sextillion. Because no one wants to write out that many zeroes, we can write the number in scientific notation this way: 6.02 x 1023 .
By the way, the number that we use to describe the amount in a mole— 602,000,000,000,000,000,000,000 or 6.02 x 1023—is known as Avogadro’s number. It is used so often in chemistry that most people end up memorizing it. It’s one of those numbers, like pi, that seems to lodge itself in the recesses of the brain and can be recalled years (even decades) after learning it.
Why is the Mole Useful?
But let’s back up and answer the question I’m sure you’re wondering: why would anyone ever need to know how to count 6.02 x 1023 things?
Remember what we’re dealing with. In chemistry, we consider the reactions that take place with atoms and molecules.
Atoms and molecules are incredibly small. Because they’re so small, it’s unreasonable to conduct a chemical reaction between single atoms or molecules. Instead, we deal in quantities of atoms and molecules that we can measure. But the number of atoms and molecules in the quantities we are able to measure is very large.
Consider hydrogen, the first element in the periodic table.
The mass of a single hydrogen atom is approximately 1.67 x 10 -24 grams (0.00000000000000000000000167 grams). That’s going to be really hard to measure.
But, if you were to weigh out just one gram* of hydrogen, how many hydrogen atoms do you think you’d have?
Well it turns out that you’d have approximately 602,000,000,000,000,000,000,000, or one mole of hydrogen atoms. So while it’s nearly impossible to weigh out a single hydrogen atom, it’s quite easy to measure out a mole of hydrogen atoms.
Using the Periodic Table to Calculate Moles
The cool thing is that you can use the periodic table to determine how much mass of each element you need to have a mole of that element. (I’ve said it before, and I’ll say it again: the periodic table has so much information packed inside!)
On the periodic table, each element is often listed with two numbers. The atomic number (the smaller of the two numbers) corresponds to the element’s order in the periodic table and tells how many protons an atom of that element possesses.
The atomic mass tells us how many grams of that element will contain a mole of atoms.
So with a simple glance at the periodic table, we can discover how much of each element is needed to have 602,000,000,000,000,000,000,000 atoms of that element.
An important thing to notice is that the amount of an element (in grams) that it takes to have a mole of atoms varies greatly.
It takes only 1.01 grams of hydrogen to have a mole of hydrogen atoms, but it takes 12 grams of carbon to have the same number of carbon atoms.
But this isn’t really so surprising when you remember that a mole is just another word that stands for a number (like pair, dozen, or gross). We can have a pair of shoes and we can have a pair of elephants: we have two of each (that’s what pair means), but the two pairs definitely don’t weigh the same.
In the same way, a mole of hydrogen atoms weighs only 1.01 grams while a mole of lead weighs 207.2 grams. But 1.01 of hydrogen and 207.2 grams of lead both contain 6.02 x 1023 atoms.
How the Mole is Used in Chemistry
In chemical reactions, the atoms or molecules that react with one another do so in certain ways. Consider the reaction between sodium (Na) and chlorine (Cl) to produce table salt (NaCl):
This is the balanced chemical equation for the formation of table salt. If you’re wondering what the numbers mean, I’d love to explain it (but don’t want to get lost down a rabbit hole). Just take my word for it that this is what the reaction looks like.
In this reaction, two atoms of sodium (Na) combine with two atoms of chlorine (shown as Cl2) to make two molecules of sodium chloride (NaCl, table salt). That’s the atomic “recipe” for making table salt from sodium and chlorine. We can also represent it like this:
Just like a recipe for food, this atomic recipe can be scaled up. Two moles of sodium atoms can combine with two moles of chlorine atoms to make two moles of sodium chloride.
And while we can’t measure out single atoms of sodium or chlorine, we CAN measure a mole of each. Looking at the periodic table, we can see that it takes 23 grams of sodium to have a mole of sodium atoms, so in 46 grams we’d have two moles. It takes 35.5 grams of chlorine to have a mole of chlorine atoms, so in 71 grams we’d have two moles. So in our scaled up version of the atomic recipe, two moles of sodium can react with two moles of chlorine to make two moles of sodium chloride. (**Do not try at home.** )
So you see, the ability to determine how many atoms are in a measured amount of an element is incredibly useful in chemistry. Knowing the atomic recipes for chemical reactions, and how many grams of each element are needed can save time and money, especially for elements that are expensive or in limited supply.
Ways to Explore the Mole Concept at Home
If your students are learning chemistry, they will need to understand the chemistry mole. My advice is to give them as many ways as you can to help them model this abstract concept.
-Have them use the periodic table to figure out how many grams of each element it takes to have a mole of that element.
-Experiment! You know that the formula for water is H2O. That means that one molecule of water is made up of two atoms of hydrogen (H) and one atom of oxygen. Scaling up, that means that one mole of water contains two moles of hydrogen atoms and one mole of oxygen atoms. How much would that one mole of water weigh? Use the periodic table to figure out how many grams in two moles of hydrogen and add to that the number of grams in one mole of oxygen. You should have just calculated that one mole of water would weigh approximately 18.02 grams. If you have a digital scale***, you can measure out 18 grams of water and know that you have 602,000,000,000,000,000,000,000 water molecules. Isn’t that cool?!
You could do a similar experiment to measure out a mole of table salt (NaCl, made up of one mole of Na and one mole of Cl).
If you’re really feeling ambitious, you can figure out how many grams it would take to have a mole of table sugar. The formula for table sugar (sucrose) is C12H22O11. That means one mole of table sugar contains 12 moles of carbon, 22 moles of hydrogen, and 11 moles of oxygen. Use the periodic table to figure out the rest.
*If you’re not familiar with how much a gram weighs, it’s about equivalent to the weight of one standard paper clip.
*** If you don’t have a scale, but have a way to measure in milliliters, 18 milliliters of water is equivalent to 18 grams of water. 18 milliliters is about 1 and one quarter tablespoons.