How many moles of magnesium are in 1.25 x 10^23 ions of magnesium?

Iknow the answer is a but i don't know why

Which statement accurately represents the arrangement of electrons in bohr’s atomic model?

)in the deacon process for the manufacture of chlorine, hcl and o2 react to form cl2 and h2o. sufficient air (21 mole% o2, 79% n2) is fed to provide 35% excess oxygen, and the fractional conversion of hcl is 85%. calculate the mole fractions of the product stream components.