When a sample of Mg(s) reacts completely with O2(g), the Mg(s) loses 5.0 moles of electrons. How many moles of electrons are gained by the O2(g)?

Explanation:

2Mg + O₂  = 2MgO .

2 moles of Mg reacts with one mole of O₂

2 moles of Mg loses 4 moles of electrons because there are 2 positive charges on each atom of Mg .

On each atom of oxygen  there are 2 negative charges

on two atom of oxygen there are 4 negative charges

on one mole of O₂ there are 4 moles of negative  charges

Hence ,

when 2 moles of Mg loses 4 moles of electrons , 4 moles of negative charges are gained by oxygen.

when 2.5  moles of Mg loses 5 moles of electrons , 5 moles of negative charges are gained by oxygen.