A piece of magnesium ribbon weight 24g is burn in air an 40g magnesium oxide is formed. What mass of oxygen combined with the magnesium in this reaction

Answer 1

24 g of Mg is how many mols of Mg ?

so How many mols of O ? (Hint: MgO has exactly as many mols of O as of Mg , note of O at 16 g/mol, not O2 at 32 g/mol)

so mass of O = mols of O * 16 g/mol