Two separate titrations of acetic acid with sodium hydroxide are performed at 25 °C…
CHEMISTRY · Multi-part · AP · Chemistry · Acids and Bases · Acids and Bases
Problem
Two separate titrations of acetic acid with sodium hydroxide are performed at 25 °C under identical conditions except for the initial acid concentration.
• Titration I: 25.0 mL of 0.100 M CH3COOH titrated with 0.100 M NaOH.
• Titration II: 25.0 mL of 0.010 M CH3COOH titrated with 0.100 M NaOH.
For each titration, the following experimental observations are made for the pH change near the equivalence point:
– Titration I: pH rises from 6.0 to 8.0 as the added NaOH volume increases from 24.5 mL to 25.5 mL.
– Titration II: pH rises from 6.4 to 7.6 as the added NaOH volume increases from 24.5 mL to 25.5 mL.
Answer the questions that follow.
(a) Calculate the average slope (change in pH per millilitre of NaOH added) over the 1.0 mL interval straddling the equivalence point for each titration.
(b) Explain, in terms of buffer capacity and acid concentration, why the slopes calculated in part (a) differ.
(c) Considering phenolphthalein (pH 8.2–10.0) and bromothymol blue (pH 6.0–7.6), discuss which indicator would provide the sharper end-point for each titration and justify your reasoning.
(d) If the concentration of acetic acid in Titration I were doubled while keeping the titrant unchanged, predict qualitatively how the slope near the equivalence point would change and explain your prediction.