[tex]6,02*10^{23} cz. MgO = 40g\\10^{12} cz. MgO = x\\x = \frac{40*10^{12} }{6,02*10^{23} }[/tex]
x = 6,644518272*[tex]10^{-11}[/tex] g = 6,645*[tex]10^{-11}[/tex] g