[Mnemonic]

I'm so confused about avogadro's number.

How were scientists able to change from oxygen and hydrogen to Carbon-12? Couldn't any number be avo's number, why did they choose 6.02 etc.

If aliens had a planet like ours and were studying chemistry, would a mole be considered 6.022 x 10^23?

[Borek]

I'm so confused about avogadro's number.

How were scientists able to change from oxygen and hydrogen to Carbon-12? Couldn't any number be avo's number, why did they choose 6.02 etc.

I don't think oxygen was ever part of the definition. It was definitely used as a part of the relative atomic mass definition, perhaps you mixed these two things.

It all starts with equivalents ("how much substance reacts with 1 gram of hydrogen"), so the path to the conclusion can be a bit bumpy. Historically what defined the Avogadro's number was the ratio of masses - mass of a mole of a substance to mass of the single molecule/atom. When you take a different substance, mass of a mole changes, but so does the mass of a single molecule/atom, and the ratio remains constant.

Later it turned out it is more convenient to use Avogadro's number to define a mole, so the definition has changed and whole system looks a bit circular.

If aliens had a planet like ours and were studying chemistry, would a mole be considered 6.022 x 10^23?

No. It is quite possible they would use similar system in which amount of substance is based on the number of atoms/molecules, but choice of the mass unit is completely arbitrary. Remember, "1 g of hydrogen" is where it started, and original definition of "1 g" was based on the Earth size and selection of water as a reference substance. While I can imagine aliens choosing water for a reference, chances that their planet has exactly the same size as Earth are slim.

[Enthalpy]

Even on Earth, different people take different units to fit their needs better. 40 000 000 m to define the metre wasn't particularly good for navigation, unless sailors had switched to the foreseen grades (400 in a turn) to measure the angles. If measuring angles in degrees, the nautical mile (360*60 = 21 600 around the Earth) is easier.

Imagine now the gram being the mass of (nm/100 000)³ instead of (km/100 000)³ water: this would have changed Avogadro's number.