Aim for FiveChemistry

Long free-response question

Titrating an unknown weak acid

  • Units 4 and 8
  • 10 points
  • About 23 minutes

You can use a calculator on this question, just like on exam day.

A multipart question, often built around a lab setup or a data table, that mixes several topics. You calculate, explain and justify your answers across many lettered parts. On the exam: 3 questions (Questions 1–3), part of the 1-hour-45-minute free-response section; calculator allowed.

The question and its sources

A student titrates a solution of an unknown monoprotic weak acid, HA, with a standardized solution of NaOH while monitoring the pH with a calibrated pH meter.

Titration procedure and results

The student titrates 25.00 mL of the HA solution with 0.1000 M NaOH at 25 °C.

The equivalence point is reached after 18.60 mL of NaOH has been added.

After 9.30 mL of NaOH had been added, the pH of the mixture was 4.87.

Source: Hypothetical data

Table 1. Acid-base indicators

IndicatorpH range of color change
Methyl orange3.1–4.4
Bromothymol blue6.0–7.6
Phenolphthalein8.2–10.0

Source: Standard indicator ranges, rounded

Suggested time: 23 minutes

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Part (a)

1 point

Write the balanced net ionic equation for the reaction that occurs between HA(aq) and NaOH(aq).

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Part (b)

1 point

Calculate the initial concentration of HA in the solution.

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Part (c)

2 points

Determine the value of Ka for HA. Justify your method.

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Part (d)

1 point

Calculate the pH of the HA solution before any NaOH was added.

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Part (e)

1 point

Is the pH at the equivalence point greater than 7, less than 7, or equal to 7? Justify your answer.

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Part (f)

1 point

Which indicator in Table 1 is most appropriate for this titration? Justify your choice.

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Part (g)

1 point

At one point during the titration, the pH of the mixture is 6.00. At this point, which is greater, [HA] or [A⁻]? Justify your answer.

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Part (h)

1 point

The student repeats the titration using 25.00 mL of 0.0744 M HCl instead of the HA solution. Will the volume of 0.1000 M NaOH needed to reach the equivalence point be greater than, less than, or equal to 18.60 mL? Justify your answer.

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Part (i)

1 point

Describe the titration curve you would draw for the titration of HA with NaOH, from 0 mL to 30 mL of NaOH added. Include the variable and unit on each axis, the approximate pH at the start, the shape of the curve before the equivalence point, and the location and approximate pH of the equivalence point.

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