The Graphs Labeled (A) And (B) Show The Titration Curves For Two Equal-Volume Samples Of Bases, One Weak And One Strong. Both Titrations Were Carried Out With The Same Concentration Of Strong Acid. (I) What Is The Approximate PH At The Equivalence Point Of Each Curve? (Ii) Which Graph Corresponds To The Titration Of The Strong Base And Which One To The
The graphs labeled (a) and (b) show the titration curves for two equal-volume samples of bases, one weak and one strong.
Both titrations were carried out with the same concentration of strong acid.
(i) What is the approximate pH at the equivalence point of each curve?
(ii) Which graph corresponds to the titration of the strong base and which one to the weak base?
The Correct Answer and Explanation is :
In titration experiments involving equal volumes of a weak base and a strong base, both titrated with the same concentration of strong acid, the resulting pH curves exhibit distinct characteristics that allow for their differentiation.
(i) Approximate pH at the Equivalence Point:
- Titration of a Strong Base: In the titration of a strong base (e.g., sodium hydroxide, NaOH) with a strong acid (e.g., hydrochloric acid, HCl), the equivalence point occurs when stoichiometrically equivalent amounts of acid and base have reacted. Given that both the acid and base are strong, the resulting solution primarily contains neutral salts (e.g., NaCl) and water. Consequently, the pH at the equivalence point is approximately 7.
- Titration of a Weak Base: When titrating a weak base (e.g., ammonia, NH₃) with a strong acid, the equivalence point is reached when all the weak base has reacted with the acid. However, the conjugate acid formed (e.g., ammonium ion, NH₄⁺) is weakly acidic and undergoes hydrolysis, releasing H⁺ ions into the solution. This results in a solution with a pH less than 7 at the equivalence point.
(ii) Identification of the Corresponding Graphs:
Analyzing the titration curves:
- Graph (A): If the pH at the equivalence point is approximately 7, the curve likely represents the titration of a strong base with a strong acid.
- Graph (B): If the pH at the equivalence point is less than 7, the curve likely represents the titration of a weak base with a strong acid.
Explanation:
The shape and characteristics of titration curves are influenced by the strengths of the acid and base involved:
- Strong Base with Strong Acid: The initial pH is high due to the strong base. As the acid is added, the pH decreases gradually until it drops sharply near the equivalence point, reflecting the rapid neutralization. Beyond the equivalence point, the pH levels off as excess acid is added.
- Weak Base with Strong Acid: Starting with a moderately high initial pH, the addition of acid leads to a buffer region where the pH changes relatively slowly. Near the equivalence point, the pH decreases more gradually compared to the strong base titration, and after the equivalence point, the pH continues to decrease slowly as excess acid is added.
By examining the pH values at various stages of the titration and the overall shape of the curve, one can distinguish between titrations involving strong and weak bases.