For every mole of any substance or element, the total number of atoms is 6.022 x 10^23. This is called the avogadro's number. Hence, for a) for a 0.90 mole copper, the number of atoms is 5.42 x10^23. This also applies for B) which gives silver as given. For c), the number of molecules present in a compound is also the same. Thus, for a,b,and c, the answer is 5.42 x10^23.
