The mole and Avogadro's number | Atoms, compounds, and ions | Chemistry | Khan Academy

Summary

The video script demystifies the chemistry concept of a mole, emphasizing it as a quantity equating to 6.02 x 10^23 entities, known as Avogadro's number. It clarifies that a mole simplifies the conversion between atomic mass units and grams, using carbon-12 as a reference substance. The script also illustrates the vast scale of atoms in a mole through analogies, likening a single atom's size to an apple and the entire apple to Earth, aiming to build intuition around moles for further chemical calculations.

Takeaways

• 🧪 A mole in chemistry is a unit that represents a specific number of particles, which is 6.02 x 10^23, also known as Avogadro's number.
• 🌟 The concept of a mole helps to bridge the gap between atomic mass units and grams, allowing for the measurement of substances in a macroscopic scale.
• 🔍 The mole is defined as the amount of substance that contains as many elementary entities as there are atoms in 12 grams of carbon-12.
• 📏 Understanding moles is crucial for translating between atomic world measurements (atomic mass units) and everyday world measurements (grams).
• ⚖️ The mass of one mole of any substance is numerically equal to its atomic or molecular mass in atomic mass units (amu).
• 🍎 If you take an apple and scale it up such that one of its atoms is the size of the apple, then the apple would be the size of the Earth, illustrating the vast number of atoms in a mole.
• 💡 The mole concept is not just a number but a practical tool for calculating the mass of a collection of atoms or molecules based on their atomic or molecular mass.
• 🔑 The script emphasizes the importance of developing an intuitive understanding of moles to avoid confusion in more advanced chemistry topics, such as energy calculations per mole.
• 📚 The video suggests that students should practice using the mole concept to enhance their comprehension and avoid difficulties in future chemistry studies.
• 🤔 The script hints at potential follow-up content, such as a video on Avogadro's number, to further clarify the concept of moles.

Q & A

What is the definition of a mole in chemistry?

A mole in chemistry is a unit that represents a specific number of entities, which is 6.02 times 10 to the 23, also known as Avogadro's number. It's a way to count atoms, molecules, or other particles in a substance.

Why is Avogadro's number significant in chemistry?

Avogadro's number is significant because it represents the number of particles (atoms, molecules, ions, etc.) in one mole of a substance, which is a fundamental concept in chemistry for calculations involving quantities of substances.

How is the mole concept used to relate atomic mass units to grams?

The mole concept is used to relate atomic mass units to grams by stating that one mole of a substance contains a number of entities (atoms, molecules, etc.) equal to Avogadro's number, and this collection of entities has a mass in grams numerically equal to the substance's atomic or molecular mass in atomic mass units.

What is the mass of 1 mole of carbon-12 atoms?

The mass of 1 mole of carbon-12 atoms is 12 grams, since carbon-12 has an atomic mass of 12 atomic mass units, and 1 mole contains Avogadro's number of such atoms.

How can you determine the mass of a mole of any element if you know its atomic mass?

To determine the mass of a mole of any element, you multiply the element's atomic mass (in atomic mass units) by Avogadro's number (6.02 x 10^23) and then convert the result to grams, since 1 mole is equivalent to the element's atomic mass in grams.

What does the Wikipedia definition of a mole imply about the relationship between moles and grams?

The Wikipedia definition implies that a mole is a measure of the amount of substance that contains as many elementary entities (like atoms or molecules) as there are atoms in 12 grams of carbon-12, which is a way to link the atomic scale to the macroscopic scale using grams.

Why is the mole concept important for understanding chemical reactions?

The mole concept is important for understanding chemical reactions because it allows chemists to quantify the amounts of reactants and products involved in a reaction, which is essential for balancing chemical equations and calculating reaction stoichiometry.

Can you provide an example of how to calculate the mass of 2 moles of aluminum, given that aluminum has an atomic mass of approximately 27?

The mass of 2 moles of aluminum can be calculated by multiplying the atomic mass of aluminum (27 amu) by Avogadro's number (6.02 x 10^23 atoms/mole) and then by 2 to account for the 2 moles, resulting in a mass of 54 grams.

What is the significance of the mole in the context of the atomic world versus the everyday world?

The mole is significant as it bridges the atomic world, where quantities are measured in atomic mass units, and the everyday world, where mass is measured in grams. It provides a practical way to translate between these two scales, facilitating measurements and calculations in chemistry.

How does the concept of a mole help in visualizing the vast number of atoms in a small mass?

The concept of a mole helps in visualizing the vast number of atoms in a small mass by providing a tangible reference point. For example, understanding that 1 mole of any substance contains Avogadro's number of atoms allows one to grasp that even a small mass like 1 gram of hydrogen contains a huge number of atoms.

Outlines

00:00

🔬 Understanding the Concept of a Mole in Chemistry

The paragraph introduces the concept of a mole in chemistry, which is a unit representing a specific number of particles, 6.02 times 10 to the 23rd power, also known as Avogadro's number. It clarifies that this is different from the common understanding of 'mole' as a small mammal or a blemish. The mole in chemistry serves as a bridge between atomic mass units and grams, allowing chemists to quantify the number of atoms or molecules in a substance. The video explains that one mole of carbon-12 atoms, which has an atomic mass of 12, weighs exactly 12 grams, illustrating the direct relationship between a mole and the mass of an element. The speaker suggests that understanding moles is crucial for grasping more complex chemical concepts and encourages viewers to practice this concept to develop intuition.

05:01

🧪 Applying the Mole Concept to Different Elements

This paragraph delves deeper into the practical application of the mole concept. It explains that the mass of one mole of any element directly corresponds to its atomic mass number in grams. For instance, one mole of hydrogen, with an atomic mass number of 1, weighs 1 gram. The speaker uses aluminum (with an atomic mass number of 27) as an example to show that one mole of aluminum atoms weighs 27 grams. The concept is further expanded to include calculations involving multiple moles of elements, such as two moles of silicon (atomic mass number 28) weighing 56 grams. The paragraph emphasizes the importance of understanding moles to comprehend chemical reactions and energy calculations, which often involve energies per mole. The speaker encourages viewers to practice using moles to develop a strong foundation in chemistry.

Keywords

💡Mole (chemistry)

In chemistry, a mole is a unit that represents the amount of a substance, defined as the number of atoms in 12 grams of carbon-12, which is approximately 6.02 x 10^23 entities. This concept is central to the video as it helps to bridge the gap between atomic mass units and macroscopic quantities like grams. The video emphasizes that a mole is not a measure of weight but rather a count of entities, such as atoms or molecules.

💡Avogadro's number

Avogadro's number, approximately 6.02 x 10^23, is the number of atoms or molecules in one mole of a substance. It is named after the Italian scientist Amedeo Avogadro and is a fundamental constant in chemistry. The video uses Avogadro's number to explain the scale of a mole, illustrating the vast number of particles that constitute a mole.

💡Atomic mass units (amu)

Atomic mass units (amu) are units used to express the mass of atoms relative to the most abundant isotope of carbon-12, where one amu is defined as 1/12th of the mass of a carbon-12 atom. The video explains how atomic mass units are used to relate the mass of atoms to the concept of moles, showing that one mole of an element with a specific atomic mass will have a mass in grams equal to that atomic mass.

💡Carbon-12

Carbon-12 is a specific isotope of carbon with an atomic mass of 12 amu. The video uses carbon-12 as a reference point to define the mole, stating that 12 grams of carbon-12 contains one mole of carbon atoms, which is Avogadro's number of atoms.

💡Elemental entities

Elemental entities refer to the individual atoms or molecules that make up a substance. The video script mentions that a mole is defined as the amount of substance that contains as many elemental entities as there are atoms in 12 grams of carbon-12, emphasizing the counting of individual units within a mole.

💡Mass number

The mass number of an element is the total number of protons and neutrons in its nucleus. In the context of the video, the mass number is used to determine the mass of a mole of a substance, as one mole of an element with a mass number of 'x' will have a mass of 'x' grams.

💡Isotopes

Isotopes are variants of a particular chemical element which differ in neutron number, and consequently in nucleon number. The video touches on isotopes by discussing iron, which has multiple isotopes, but for simplicity, focuses on iron-56, an isotope with a mass number of 56.

💡Gram

A gram is a unit of mass in the metric system and is defined as one-thousandth of a kilogram. The video uses grams to illustrate the mass of a mole of a substance, explaining that the mass of one mole of an element is numerically equivalent to its atomic mass in grams.

💡Translation (between atomic and macroscopic scale)

The video emphasizes the importance of the mole as a tool for translation between the atomic scale, where mass is measured in atomic mass units, and the macroscopic scale, where mass is measured in grams. This translation is crucial for understanding chemical reactions and calculations involving quantities of substances.

💡Intuition

Intuition in the context of the video refers to the ability to understand and apply the concept of moles without needing to rely on formulas or definitions. The speaker encourages viewers to develop an intuitive grasp of moles to better comprehend more complex chemical concepts, such as energy changes in reactions measured per mole.

Highlights

A mole in chemistry is a concept that confuses many students, yet it is one of the simplest concepts.

A mole is a number, specifically 6.02 times 10 to the 23, also known as Avogadro's number.

Wikipedia defines a mole as the amount of substance that contains as many entities as there are atoms in 12 grams of carbon-12.

One mole of carbon-12 atoms is equivalent to 12 grams of carbon, illustrating the mole's utility.

A mole is a bridge between atomic mass units and grams, aiding in the conversion between atomic and macroscopic scales.

The mass of a mole of an element is directly related to its atomic mass number in atomic mass units.

For example, 1 mole of hydrogen, with an atomic mass number of 1, has a mass of 1 gram.

The concept of a mole is essential for understanding chemical reactions and their associated energies.

The mole is a fundamental unit for translating between atomic mass units and grams.

Understanding moles is crucial for grasping chemical calculations and reactions.

The mole concept allows for the conversion of atomic mass units to grams, simplifying chemical calculations.

Visualizing the scale of a mole is challenging due to the vast number of atoms in a small mass, such as 1 gram of carbon.

The mole is a way to quantify the number of atoms or molecules in a given mass of a substance.

The mole is used to express the amount of substance in chemical equations and reactions.

The concept of a mole is central to stoichiometry and understanding the relationships between reactants and products in chemical reactions.

The mole is a unit that helps chemists work with the vast numbers of particles involved in chemical processes.

The mole is a key concept in chemistry that connects the microscopic world of atoms to the macroscopic world of grams and kilograms.

Transcripts

Let's talk about a concept that probably confuses

chemistry students the most on some level.

But on some level it's also one of the simplest concepts.

And that's the idea of a mole, which in chemistry is

different than the thing digging up your backyard, or

the thing you want to get removed from your left eye.

A mole in chemistry is just a number.

It's just a number, and the number is 6.02

times 10 to the 23.

So it's a very huge number.

And this is also called Avogadro's number.

Maybe I will do a video on Avogadro.

But that's all you need to know.

A mole is just a number.

There are kind of more Byzantine

definitions of a mole.

This actually is not-- actually, let me copy and

paste it from Wikipedia.

This is Wikipedia's definition of a mole.

And you hopefully at the end of this video you'll see that

they're equivalent.

But if you're just getting exposed to the concept, this

to me, it's just not an easy concept.

Basically, a "a mole is defined as the amount of

substance of a system that contains as many elemental

entities as there are atoms in 12 grams of carbon 12." Well,

I just told you that a mole is 6.02 times 10 to the 23.

So if you just take the last part, atoms in 12

grams of carbon 12.

So that means that there are 1 mole of carbon 12-- let me

write it like that-- carbon 12.

There are 1 mole of carbon 12 atoms in 12 grams of carbon.

And so that's why a mole is useful.

So I could have just instead of writing 1 mole, I could

have replaced this as there's 6.02 times 10 to the 23 carbon

atoms, carbon 12 atoms in 12 grams of carbon.

How do you figure that out?

Or I guess, what else does this mean?

I mean, we just added in carbon, they said it's the

amount of substance of any molecule, if you convert

between atomic mass units and grams.

This I find very confusing.

How can we apply this in other places?

So the first thing to realize is a mole is just a way of

translating between grams and atomic mass units.

One carbon 12 atom is what?

What's its mass number?

It's 12.

That's why it's called carbon 12 instead of carbon 14.

So its mass is 12 atomic mass units.

So if you have something that has a mass of 12 atomic mass

units and you have a mole of them, or you have 6.02 times

10 to the twenty three of them, all of those atoms

combined will have a mass of 12 grams.

So another way to think about it is 1 gram is equal to 1

mole of atomic mass units.

I'll write amu's like that.

Or you can write 1 gram is equal to 6.02 times 10 to the

23 atomic mass units.

And the reason why this is useful-- and it's kind of

addressed in this Wikipedia definition there-- is it helps

us translate between the atomic world-- where we deal

with atomic mass units and we deal with, oh, we've got an

extra neutron now, let's add one to our atomic mass

number-- and translating between that atomic world and

our everyday world where we deal in grams.

And just so you know, a gram is still a pretty

small amount of mass.

It's 1/1,000 of a kilogram.

A kilogram is about 2 pounds.

So this is about 1/500 of a pound.

So this is not much.

So there's a ton of atoms in a very small amount of-- in 1

gram of carbon, or at least in 12 grams of carbon, you have a

ton of atoms. You have 6.02 times 10 to the 23.

And just to hit the point home, I probably should have

talked about this in the atom.

This is a huge number.

To maybe visualize it, if you think of-- I was told that in

the diameter of a hair, if this is a hair and this is

diameter of the hair, if you go this way there 1 million

carbon atoms. 1 million carbon atoms that way.

Or if you were to take an apple and you were to try to

figure out what fraction, if you were to make one of the

atoms of an apple-- and obviously, an apple has a

bunch of different types of atoms in it-- but if you were

to take one of the atoms and make it the size of the apple,

then the apple would be the size of the earth.

So an apple atom is to an apple as an

apple is to the earth.

So these are obviously-- it's hard for us to even process

things of this size.

When you just have one gram of-- well, let's say you have

1 gram of hydrogen.

1 gram of hydrogen.

If you have 1 gram of hydrogen, that means you have

1 mole of hydrogen.

How do I know that?

Because hydrogen's atomic mass number is 1.

So in general, if you just take any element-- so what is

the mass of, let me just pick, 1 mole of aluminum?

So if I were to take 6.02 times 10 to the 23 aluminum

atoms, what is the mass of that collection?

Well, each of them have an atomic mass number of 13.

So it's 13 amu's-- I don't have to put the s there--

times six point-- well, I won't

write that way, actually.

That'll probably just confuse you.

The easy way to think about is if you have a mole of an atom,

you take its mass-- I was taking its atomic number,

that's not good-- you take its mass number.

In this case let's say it's 27.

So we're dealing with aluminum 27.

You take its mass number, and if you have 1 mole of it, then

the mass of that will be 27 grams.

So that literally, when you have one mole of an atom it's

a direct translation between its mass number and grams. 1

mole of iron, let's say iron 56-- there's obviously many

isotopes or iron-- let's say we're dealing with iron 56.

You normally don't hear it like that, but let's say we're

dealing with the isotope of iron that has a

mass number of 56.

So if I have 1 mole of this, 1 mole of this atom right here,

that's going to have a mass of-- the math isn't difficult

here-- 56 grams. And if you think about it, how many

atomic mass units is this?

Well, this is 56 atomic mass units per atom.

Then you have a mole of those, so you have 6.02 times 10 to

the 23 times 56 atomic mass units.

And then you divide it by the number of atomic

mass units per gram.

And you end up with 56 grams.

But the easy way to think about it is you just take

whatever the mass number is.

If you have silicon, if you have a mole of silicon, a mole

of silicon will have a mass-- I don't want to say weight

because this should apply to any planet-- of 28 grams.

What about 2 moles of silicon?

And I'll write its mass number.

Let's say silicon has a mass number of 28.

Two moles of silicon.

Well, 1 more would have a mass of 28 grams, so 2 moles is

going to have a mass of 56 grams.

If I were to say, let's say I had 4 moles of oxygen, which

has a mass number of 16.

What is the mass of that?

This is a huge number of oxygen atoms-- what would be

the mass of that?

Well, it would be 4 times-- 1 mole of oxygen would have a

mass of 16 grams, so 4 moles has 64 grams.

It's confusing because we're not used to using a word like

moles as a number, but all it is is a number.

And the easy way to think about is that it lets us

translate between this atomic mass unit number and grams.

And you say, well, how do I get that many grams?

Well, I have to have 6.02 times 10 to the 23 carbon

atoms for that collection of carbon to have a mass of 12

grams. That's all that mole means.

It's just a number.

And I encourage you to kind of play around with a lot of what

we talked about.

Because it's super important to have the intuition behind

moles, otherwise you'll get confused later on when we

start getting into energies in terms of it requires

kilojoules per mole, and what is the energy of this reaction

and all that type of stuff.

So just really try to make sure you digest this as well

as possible.

And let me know if you don't and I'll maybe make another

video on this because it's so important.