Final answer:
To determine the mass of the excess reactant in a reaction between magnesium and phosphoric acid producing magnesium phosphate, one must perform a mass-mass calculation using the balanced equation, subtract the mass of the product from the mass of the reactants, and then identify and calculate the remaining mass of the excess reactant.
Step-by-step explanation:
When 46 grams of magnesium reacts with 132.75 grams of phosphoric acid to produce 90.45 grams of magnesium phosphate, the mass of the excess reactant can be determined by subtracting the mass of the product from the total mass of the reactants used, according to the law of conservation of mass. First, one would need to use the balanced chemical equation to ascertain which reactant is the limiting reactant and which is in excess. Since magnesium is reacting with phosphoric acid to form magnesium phosphate, the balanced equation would help us understand the stoichiometry of the reaction and thus the theoretical yield.
Typically, a mass-mass calculation would be done to determine how much of each reactant is required to produce the product, but we can use the information provided directly. The initial masses of magnesium and phosphoric acid are given, as well as the mass of the product. By subtracting the mass of the product from the total initial mass of the reactants, one can find the mass of the reactants that did not react. From there, using the balanced chemical equation, one can deduct which reactant is in excess and calculate its remaining mass.