Answer: 55.56 moles of MgO
Step-by-step explanation:
1 mole = avogadro number
if 6.21x10^25 atoms then molecules of MgO (equal numbers of Mg and O atoms)
= 0.5*6.21x10^25 = 3.105 molecules of MgO
So moles of MgO = 0.5*6.21*10^25/avogadro number = 55.56