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## AP®︎/College Chemistry

### Course: AP®︎/College Chemistry>Unit 1

Lesson 1: Moles and molar mass

# The mole and Avogadro's number

One mole of a substance is equal to 6.022 × 10²³ units of that substance (such as atoms, molecules, or ions). The number 6.022 × 10²³ is known as Avogadro's number or Avogadro's constant. The concept of the mole can be used to convert between mass and number of particles.. Created by Sal Khan.

## Want to join the conversation?

• So if my understanding of a Mole is correct, basically it is just a tool that was create so that amu could be associated in more simple terms such as a gram? •   Absolutely correct. The mole is a conversion factor to convert from amus to grams. It's just a very big number because it takes a lot of amus to weigh one gram.

For your amusement - it is a lot easier to weigh out 1 gram of a bunch of atoms or molecules for a lab experiment than it is to find a single atom/molecule and work with it in the lab.
• i am confused......
In the starting of the video it is mentioned that 1 mole is = 1 gram
but later he said that 1 mole of iron = 56 grams as atomic mass of iron is 56 so why he said that 1 mole is = 1 gram????? •   Think of "mole" as a descriptive unit of measure, like a "dozen."
A dozen is always 12 of something - compare a dozen eggs versus a dozen textbooks, for example. In the same way, the mole always refers to 6.022x10^23 of something. So a mole of carbon versus a mole of iron still means the same number of carbon atoms and iron atoms.
That being said, just because something is equal in number doesn't mean it's equal in mass; a dozen textbooks is obviously much heavier than a dozen eggs. Mass is unique to the item you are weighing. So, in the same way, 1 mole of carbon is 12 grams because it is 12 atomic mass units on the periodic table and 1 mole of iron is 56 grams because it is 56 atomic mass units on the periodic table.
• How do you calculate the number of particles in 1.00 mole of nitrogen atoms? • Is mole a way to express the quantity of a substance? • Correct. While the mass of 1 mole of a particular substance will vary, 1 mole of ANY substance will ALWAYS have approximately 6.02x10^23 atoms (rounded to 3 significant figures).

This provides an easy way to express quantities especially when doing multi-step calculations during stoichiometry.
• I was always told that mass, the SI unit for which is the kilogram, was just a measure of how much matter is in an object. However, the mole is basically measuring just that; the amount of amus, or matter in an object. Yet, both the mole and the kilogram are SI units. What is the difference between the two? •  The kilogram is used for measuring, as you say, the mass of something and moles are used for measuring the amount of particles in something. For example 1 mol of carbon would be 12g and 1 mol of oxygen would be 16g but both would contain 6.02x10^23 particles. The difference being that mass of a oxygen atom is more than that of a carbon atom.
• what was sal telling about relation between 13 almunium and avagadro number when he stooped its making all this confusing? • I agree completely with Just Keith's answer of the over complication of this video along with his great detail on the background and mathematics of the mole and atomic mass unit.

I would just like to add maybe an even more basic explanation that I would employ to start with: when going to the grocery store to shop for you and your family, you would not pick up eggs individually anymore, or say 27 mL of milk or a specific amount of grains of salt (say 5000 grains), you would get say a donzen eggs, a litre/gallon of milk and specific mass/weight in gram/ounce of salt depending upon which unit of measure you use, this being more convenient and manageable number(s) when shopping. The last item, the salt, it would not be feasible to count out grains or even to determine the grain size being consistent, but would be more like the problems encountered for the much smaller atom/compound/molecule.

Now, for Chemistry when dealing with atoms, compounds or particles of elements, the 'convenient number' is the mole, which makes the use of the atomic mass numbers on the Periodic table manageable and much easier to use both in theoretical calculations and practical experimentation with chemicals. Then I would go into the more specific and accurate details given by Just Keith to explain the concept thoroughly.

I hope this story/example helps out as a starting point for what can be a very difficult concept depending on how it is approached.
• richard carrying this comment section fr • what does amu stand for • "Amu" stands for Atomic Mass unit.

It is the mass of one proton/neutron to 5 significant figures(1.6605 x 10^(-27) kg).
The mass of the proton and neutron vary negligibly. The no. of amus is the atomic mass number of an atom. Electrons are excluded.  