Read a titration curve and find the concentration
Three steps, the way the exam actually works: work through the lab, write down your own measurements, then answer a 6-point free response. What you recorded goes to the grader with your writing, so a conclusion that does not follow from your own numbers will cost you the point — exactly as it would with a real reader.
Run the investigation
- 1Reset the titration so no base has been added.
- 2Record the pH before any base is added.
- 3Step the base volume to 10.0 mL and record the pH.
- 4Step to 24.5 mL, then 25.5 mL, recording the pH at each.
- 5Find the volume where the curve rises most steeply and record it as your equivalence volume.
Booting the lab…
Record what you measured
These are your numbers, not ours. The grader sees them, so your conclusions have to follow from what you actually recorded.
| pH at 0.0 mL base(before any titrant) | |
|---|---|
| pH at 10.0 mL base | |
| Equivalence volume(steepest part of the curve) | mL |
| pH at 24.5 mL | |
| pH at 25.5 mL |
Answer the free response
A 25.0 mL sample of hydrochloric acid is titrated with 0.100 M sodium hydroxide, producing the curve you generated. (a) Using your equivalence volume, calculate the molarity of the hydrochloric acid. Show your setup. (b) Your pH readings at 24.5 mL and 25.5 mL differ by roughly eight pH units across only 1.0 mL of added base. Explain, in terms of moles of excess reactant, why the pH changes so violently in that interval. (c) Your equivalence pH should be very close to 7.00. Explain why, and state how the equivalence pH would differ if the acid had instead been acetic acid at the same concentration. (d) A student proposes using methyl red, which changes color between pH 4.4 and pH 6.2, as the indicator for this titration. Evaluate whether that choice would produce a significant titration error, and justify your answer using the shape of your curve.
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