calculez moliCuSO4, moli O si apoi numar de atomiO
M= 160g/mol CuSo4
n sau niu= 20g/160g/mol=0,125mol CuSO4
1mol CuSO4 contine 4moliO--->
n= 4x0,125molO
1mol particule contine 6,023x10²³particule(numarul lui Avogadro
4X0,125mol ----->N= 4x0,125x6,023x10²³ atomi O