Final answer:
To calculate the percent yield, you would divide the actual yield by the theoretical yield and multiply by 100%. Since oxygen is in excess, it is assumed that the theoretical and actual yields match, leading to a percent yield of 100%.
Step-by-step explanation:
To calculate the percent yield of magnesium oxide, you first need to know the theoretical yield. The theoretical yield is the maximum amount of product that can be formed from the given amounts of reactants. Since oxygen is the excess reactant in this reaction, our focus is on the magnesium. Using stoichiometry based on the balanced equation 2Mg + O2 → 2MgO, you would determine the amount of MgO that could be produced from the given amount of magnesium. However, the question has given us that 10 g of MgO are actually produced. To find the percent yield, you divide the actual yield (10 g MgO) by the theoretical yield and then multiply by 100%.
To find the percent yield if 20 g of O2 is the starting amount, you first need the molar mass of O2 and MgO to calculate the moles of MgO that can be theoretically produced from 20 g of O2. However, the question does not provide sufficient data to find the theoretical yield. Assuming the theoretical yield is the same as the actual yield (since oxygen is in excess and cannot limit the reaction's yield), the percent yield would be 100%.