From your Periodic Table we learn that one mole of copper, 6.022xx10^236.022×1023 individual copper atoms have a mass of 63.55*g63.55⋅g. And thus we use the MASS of a chemical sample to calculate the NUMBER of atoms and molecules. This is the principle of chemical equivalence, and you will do many of these calculations over the next few years......
Given that we know that if there are 63.55*g63.55⋅g copper there are N_ANA, "Avogadro's number"Avogadro's number of copper atoms.........we take the quotient.....
(6.4*g)/(63.55*g*mol^-1)xx6.022xx10^236.4⋅g63.55⋅g⋅mol−1×6.022×1023 "copper atoms mol"""^-1copper atoms mol−1.
-=6.022xx10^22≡6.022×1022 "copper atoms"copper atoms..............
