Question

Mg2+ is much more hydrated than Na+. why

Answer 1

you have Na+ and Mg2+, both simillar size but Mg has higher oxidation state and 1 more proton (possitive charged core) than Na+ so the ions are of the same size but charge is different. So the larger the charge, the same volume, still the larger the charge density, has much more hydrate capacity. Mg2+ ion will be able to hold more water molecules than Na+. 

Mg2+ is much more hydrate than Na+

keep learning buddy