How many moles of magnesium oxide are formed when 4 moles of magnesium react with oxygen?

2 moles

One mole of oxygen react with one mole of magnesium forming magnesium oxide: 2 Mg + O2 = 2 MgO

In 7.6 mole MgS there is 7.6 mole Mg(2+) AND 7.6 mole S(2-)

5.55 moles Mg ((6.022 × 1023) ÷ 1 mol Mg) = 3.34 × 1024 atoms of Mg

First you need to find the atomic mass of magnesium, 24.3 grams. Then you can do a simple conversion: Grams ÷ Atomic mass = Moles 7.5 grams ÷ 24.3 grams = 0.309 moles Mg