[tex]m=6,9g\\
M_{Al_{2}(SO_{4})_{3}}=342,17\frac{g}{mol}\\\\
n=\frac{6,9g}{342,17\frac{g}{mol}}\approx0,02mol\\\\
0,02mol*4*3=0,24moles \ of \ O \ atoms\\\\
6,02*10^{23} \ \ \ \ \rightarrow \ \ \ 1mol\\
x \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \rightarrow \ \ \ 0,24mol\\\\
x=\frac{0,24mol*6,02*10^{23}}{1mol}\approx1,44*10^{23}[/tex]
