CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
Benzoic acid (C₆H₅COOH, Ka=6.3×10−5 at 25 °C) is a weak monoprotic acid that is often used as a food preservative. A student titrates 40.00 mL of 0.0500 M benzoic acid with 0.100 M NaOH at 25 °C.
Three common laboratory indicators are available:
• Bromophenol blue pH 3.0 – 4.6
• Bromothymol blue pH 6.0 – 7.6
• Phenolphthalein pH 8.2 – 10.0
(a) Calculate the volume of 0.100 M NaOH required to reach the equivalence point.
(b) Determine the pH at the equivalence point. Show all supporting work.
(c) Identify which indicator from the list is most appropriate for signaling the endpoint of this titration. Justify your choice quantitatively, referring to your result from part (b) and each indicator’s transition range.