Question

How many copper atoms in a `63.55*g` mass of copper metal?

Answer 1

By definition, there are `N_A, "Avogadro's number"` of copper atoms in a `63.55*g` mass of copper wire.

Explanation:

And `N_A, "Avogadro's number"` `=` `6.022xx10^23` individual copper atoms.

I look on the Periodic Table, and at `Z=29`, i.e. copper, the quoted atomic mass is `63.55*g`. The quoted mass is the mass of `"Avogadro's number"` of copper atoms. And thus, given a reaction with, say, oxygen or sulfur, I can calculate an equivalent, STOICHIOMETRIC mass of sulfur or oxygen.

My advice is to look at the Periodic Table, and get to know it; you will soon learn the atomic masses of the common reagents.