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) ?

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.