A 25.0 mL sample of 0.200 M aqueous acetic acid, CH COOH
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A 25.0 mL sample of 0.200 M aqueous acetic acid, CH3COOH, is titrated at 25 °C with 0.200 M NaOH. A verbal description of the resulting pH–volume curve is provided to students: the initial pH is about 2.8; the curve rises gently and displays an extended, nearly flat region centred near pH 4.8; a very steep rise begins close to 24 mL of added base and ends near pH 8.7 at 25 mL; beyond 30 mL the pH levels off near 12.5.
Answer the following questions about the titration.
(a) Describe the species predominating in solution from 0 mL to 24 mL of added NaOH and explain how their presence accounts for the gradual change in pH during this interval.
(b) Identify the buffer region on the titration curve and explain, with chemical reasoning, why the solution acts as a buffer in that region.
(c) Explain why the pH at the equivalence point is greater than 7 even though exactly equimolar quantities of acid and base have reacted.
(d) A student proposes using methyl orange (transition range pH 3.1–4.4) as the indicator for this titration. State whether this choice is appropriate and justify your answer using features of the titration curve.