Answer is: the mass of the oxygen is 2.37 grams.
Balanced chemical reaction: 2Mg + O₂ → 2MgO.
m(Mg) = 3.6 g; mass of magnesium.
n(Mg) = m(Mg) ÷ M(Mg).
n(Mg) = 3.6 g ÷ 24.3 g/mol
n(Mg) = 0.149 mol.; amount of the magnesium.
n(O₂) = ?
From chemical reaction: n(Mg) : n(O₂) = 2 : 1.
n(O₂) = n(Mg) ÷ 2.
n(O₂) = 0.149mol ÷ 2.
n(O₂) = 0.075 mol; amount of the oxygen.
m(O₂) = m(O₂) · M(O₂).
m(O₂) = 0.075 mol · 32 g/mol.
m(O₂) = 2.37 g; mass of the oxygen.
