A lab manual provides the following textual description of the pH curve obtained when…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A lab manual provides the following textual description of the pH curve obtained when 0.200 M NaOH is gradually added to 25.00 mL of 0.200 M H₂SO₄ at 25 °C:
• Initial pH = 0.70 and rises slowly until 12.50 mL of NaOH have been added.
• A sharp rise in pH occurs between 12.50 mL and 25.00 mL of NaOH, after which the pH levels off near 12.
Methyl orange (pH 3.1–4.4) was the indicator used in the experiment.
(a) Identify the volume of NaOH corresponding to the equivalence point for the neutralization of the first acidic proton of H₂SO₄, and justify your answer using the stoichiometry of the reaction.
(b) Explain why only one sharp rise in pH is observed rather than two distinct jumps that might be expected for a diprotic acid.
(c) Based on the indicator’s transition range and the pH profile, discuss whether methyl orange provides an end point that coincides with the equivalence point identified in part (a).
(d) If a student repeated the titration using bromothymol blue (pH 6.0–7.6) instead, predict how the calculated concentration of NaOH would differ from the true value (too high, too low, or unchanged). Provide a thorough justification that integrates the titration curve description with the chemical properties of the indicator.