Mole Day: What the Heck Is a Mole and How Do You Measure It?

Rhett Allain
6 min readOct 23, 2024
Image: Press/NIST; graphic design: N. Hanacek/NIST — A mole of aluminum, carbon, and copper

When you take your first chemistry course, you are going to see a mole. Not the animal, the number of things. Oh, and guess what? October 23 is Mole Day. Why? It’s because a mole is 6.02 x 10²³ of something and in the USA, we write this date as 10/23. Get it? Yes, we call this the Avogadro Constant (or Avogadro’s number).

Let’s go over some of the fun and interesting things about moles.

Why Do We Use Moles?

I should point out one big differences between most (but not all) chemistry and physics. In physics, we like to think about fundamental interactions and fundamental particles. For us (I’m a physicist), we think of a gas as a bunch of particles moving around and interacting with the walls of a container. In chemistry — you can think of a gas as a continuous thing (even though everyone knows it’s not continuous).

In many ways, the chemistry approach makes more sense. Even for a very small container of air, the physics approach would have to consider 10²³ particles. That’s a LOT. For the chemistry approach, you could just deal with the pressure, volume and temperature and not worry about the individual things.

But even without know HOW many of a thing you have in a gas — sometimes it’s clear that we can have twice as many of…

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Rhett Allain
Rhett Allain

Written by Rhett Allain

Physics faculty, science blogger of all things geek. Technical Consultant for CBS MacGyver and MythBusters. WIRED blogger.

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