It’s because we use grams in chemistry.
Remember Avogadro? The guy with the number? All he did was ask, “if something has an atomic mass of 1, the how many atoms are in 1 gram of that stuff?”.
It turns out the number is 6.02 X 10^(23) atoms. but that’s a messy number, and it takes too long to say, so we just call it a mole.
But if the atomic mass is 12, then you’d need to weigh 12 grams of that stuff to get 6.02 x 10^(23) atoms.
If he had used pounds, instead of grams, he would have ended up with a different number.
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