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How it is possible that for every substance the Avogadro number is constant? I mean, how is it possible that 1 mole of Hydrogen molecule and 1 mole of Oxygen molecule have the same number? Please explain to me.
Question Date: 2014-01-23
Answer 1:

What is Avogadro’s number? Why is it a constant?

Avogadro’s number is a unit of how many of something you have. This number is very similar to the way the word “dozen” is used. If I have a dozen eggs, I have twelve eggs. If you have a dozen doughnuts, you have twelve doughnuts. In this case, the word “dozen” isn’t specially related to eggs or doughnuts, it’s just a way of counting how many I have. And no matter what the thing is you’re counting, a dozen is always equal to exactly twelve of those things. Dozen is a word used to describe a group of twelve and only twelve things; when referring to dozens, you could say that the number twelve is constant.

Similarly, Avogadro’s number is a constant number. Only instead of counting twelve things in a dozen, you count an Avogadro’s number (6.022 x 1023) of things in a mole. If I have a mole of eggs, I have 6.022 x 1023 eggs. If you have a mole of doughnuts, you have 6.022 x 1023 doughnuts and should probably go on a diet. The same goes for hydrogen and oxygen. If you could somehow count all of the H2 molecules individually in a mole of hydrogen, you would count 6.022 x 1023 of them, and if you could count the O2 molecules in a mole of oxygen individually you would also find 6.022 x 1023 of them.

So Avogadro’s number is just that. It’s just a number, nothing else.

What’s so special about Avogadro’s number?

Avogadro’s number comes from two important observations: one involving mass and the other involving chemistry.

Most of the mass of an atom comes from the nucleus, which is made up of protons and neutrons. Protons and neutrons have roughly the same mass, and this mass is the atomic mass unit (AMU), which is equal to about 1.6605 x 10-­‐24 grams.*

Atoms, in general, don’t care about mass when reacting; atoms care about electrons and bonding. Sodium doesn’t care that chlorine has an atomic mass of 35.45 when forming NaCl, it cares that chlorine has seven electrons in its outer shell. Whether dealing with a light element like fluorine or a heavy one like iodine, sodium will always bond one to one with a group VII halogen because their electronic structures all look similar.

Unfortunately, it’s very hard to count individual atoms (who has the time?) but it’s very easy to measure mass. Since atoms have mass and the number of atoms you have should be proportional to mass, then if you give me the mass of a compound, I can tell you exactly how many atoms are in it!

There are many ways one could convert between number of atoms and mass, but a convenient one is to relate the AMU to grams. How many AMU are in one gram?

1 AMU/ (1 gram × 1.6605×10-24 grams) = 6.022×1023 AMU = 1 mole AMU

Avogadro’s number relates atomic masses to grams. If you have one mole of hydrogen, which has an atomic number of 1.008 AMU, then your hydrogen will have a mass of 1.008 grams. If I have 22.99 grams of sodium, I need at least 35.45 grams of chlorine to react all of it to NaCl.

One mole of hydrogen and one mole of oxygen have the same number of molecules because a mole is a number of things, not a unit of mass. It doesn’t tell you “what” or “how much”, only “how many”.

*The atomic mass unit is technically defined as one-­‐twelfth the mass of a carbon atom.

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