Why mole is considered an arbitrary unit?

Answer 1

6.02 x 1023 atoms is an arbitrary number.

Avagodro could have simply said that a "mole" is 12 atoms, or 1x1023 atoms, and achieved a similar result.

When we say that a mole of a substance in a given reaction yields one mole of this atom and one-half mole of another we are essentially saying that for x amount of a starting chemical, we produce x amount of the first element, and 1/2 x amount of the second element.

The x amount that we use could be 1000 atoms, 10,000 atoms, or 1x10100 atoms. In any case, we will get x number of one, and 1/2 x of the other, regardless of what x is. A mole, or 6.02x1023 atoms (or molecules) is the arbitrary number that we typically use.

By contrast, acceleration due to gravity is NOT arbitrary. The mass of the earth determines that for objects near the surface of the earth, there will be an acceleration of 11.9 m/s2 regardless of what we choose to call it. So gravity is Intrinsic (forced by something else) where a mole is just a convenient (arbitrary) way to talk about a certain number of atoms (or molecules).

Answer 2

Mole is considered an arbitrary unit because it represents a specific quantity of entities (Avogadro's number) that can vary depending on the substance being measured. It is a convenient unit for chemists to use in calculations because it allows for consistent comparisons of the amounts of different substances based on their molecular weights.