Final answer:
Oxygen (O₂) is the excess reactant, with 0.08 g left after the reaction between Mg and O₂, according to stoichiometric calculations based on the balanced chemical equation and molar masses of the reactants.
Step-by-step explanation:
The student is trying to determine which reactant is left in excess after the reaction between magnesium (Mg) and oxygen (O₂) and how much of it remains. The chemical reaction is between 1.0 g of Mg and 0.28 g of O₂ to produce magnesium oxide (MgO). To find the limiting reactant and the amount of excess reactant, we first need to compare the molar ratio of the reactants to their coefficients in the balanced chemical equation for the reaction Mg + 1/2 O₂ → MgO.
Given that magnesium is the limiting reactant in this scenario, we calculate the amount of magnesium oxide that would be produced by it and subsequently find how much oxygen would react with this amount of magnesium. If the amount of oxygen available exceeds the amount needed to react completely with the magnesium, then oxygen is the excess reactant. Using molar mass calculations, we can then determine the exact mass of oxygen that was not consumed in the reaction.
After performing these calculations, it becomes clear that O₂ is the excess reactant. The mass of remaining excess reactant is found by subtracting the mass of the excess reactant consumed from the total mass of excess reactant given. This calculation shows that the correct answer is O₂, 0.08 g, which corresponds with option B.