Determine the number of moles of MgO that form from the reaction of 4.4 moles of O2 in the following unbalanced reaction:

Mg(s) + O2(g) → MgO(s)

Balance the equation.

2Mg + O2 ==> 2MgO.

The balanced equation tells you that 1 mole of oxygen will produce 2 moles of MgO. So 2 moles of oxygen will produce 4 moles MgO or 4.4 moles of oxygen will produce ???? moles MgO. It's just a simple proportion.