Final answer:
The starting masses of the reactants are 120.3 g of potassium hydroxide and 106.5 g of phosphoric acid, which is the correct answer, option (A). This was determined through stoichiometry using the balanced chemical equation and the masses of products and remaining reactants provided in the question.
Step-by-step explanation:
To determine the starting mass of each reactant in the reaction of potassium hydroxide (KOH) with phosphoric acid (H3PO4) to form potassium phosphate (K3PO4) and water (H2O), we can use stoichiometry. The balanced equation for the reaction is: 3 KOH(aq) + H3PO4(aq) → K3PO4(aq) + 3 H2O(l).
From the question, we know that 58.5 g of K3PO4 is produced, and 89.3 g of H3PO4 remains unreacted. To calculate the starting mass of KOH, we first find the molar mass of K3PO4 and convert the given mass to moles. Then, using the molar ratio from the balanced equation, we determine the moles of KOH needed and convert it back to grams.
Since H3PO4 is in excess, we subtract the mass of the unreacted H3PO4 from the total starting mass of H3PO4 to find the mass that actually reacted. The total starting mass of both reactants can be found by combining the mass of KOH that reacted and the total starting mass of H3PO4.
Option (A) 120.3 g KOH, 106.5 g H3PO4 is the correct answer after performing the stoichiometric calculations.