[tex]m_{O_2}=3\ kg=3\ 000 \ g \\ M_{O_2}=16 \cdot 2=32 \ g/mol \\ \nu=\frac{m}{M}=\frac{n}{N_A} \\ \Rightarrow n=\frac{m_{O_2} \cdot N_A}{M_{O_2}}=\frac{3 \ 000 \ g \cdot 6,02 \cdot 10^{23} \ molecule/mol}{32 \ g/mol} \\ \Rightarrow \boxed{n\approx 5,64 \cdot 10^{25} \ molecule}[/tex]
