Which term describes the process used to determine the unknown concentration of an acid or base by reacting it with a solution of known concentration?

• A. Amphoteric species
• B. Buffer Solution
• C. Titration
• D. pH

Answer: (C)

Titration is the quantitative method used to determine the unknown concentration of an acid or base by reacting it with a solution of known concentration. In practice, you deliver the known-concentration solution (the titrant) from a burette into the analyte until the reaction is complete, which is reached at the equivalence point when the stoichiometrically required amounts have reacted. At that moment you know how many moles of titrant were used, and from the reaction’s stoichiometry you can calculate the unknown concentration of the analyte. For a simple acid–base pair, the relation M1V1 = M2V2 (adjusted for the actual stoichiometric coefficients) gives the unknown concentration. For example, titrating 25.0 mL of an unknown HCl solution with 0.100 M NaOH requires 30.0 mL of NaOH to reach the end point, so the HCl concentration is (0.100 M × 30.0 mL) / 25.0 mL = 0.120 M. Other terms like pH, buffer solution, or amphoteric species describe properties or states rather than the method of determining concentration, so they don’t describe the process.