The balanced chemical equation for the reaction is as follows; Ca(OH)2 + 2HCl —> CaCl2 + 2H2O The stoichiometry of Ca(OH)2 to HCl is 1:2. Number of Ca(OH)2 moles reacted = 0.00100 mol Therefore number of HCl moles that should react with base is double the amount of moles of base reacted. Number of acid moles in 25.00 ml = 0.00100 mol x2 = 0.00200 mol Correct answer is 0.002 mol