Long free-response question

Titrating an ammonia solution

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 determines the concentration of a solution of ammonia, NH₃, by titrating it with hydrochloric acid while monitoring the pH. For NH₃, Kb = 1.8 × 10⁻⁵ at 25 °C.

Titration procedure and results

The student titrates 25.00 mL of the NH₃ solution with 0.150 M HCl at 25 °C.

The equivalence point is reached after 20.40 mL of HCl has been added.

Source: Hypothetical data

Table 1. Acid-base indicators

Indicator	pH range of color change
Methyl orange	3.1–4.4
Methyl red	4.4–6.2
Phenolphthalein	8.2–10.0

Source: Standard indicator ranges, rounded

Suggested time: 23 minutes

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Part (a)

1 point

Calculate the pH of the 0.150 M HCl titrant.

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Part (b)

1 point

Write the balanced net ionic equation for the reaction between NH₃(aq) and HCl(aq).

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Part (c)

1 point

Calculate the initial concentration of NH₃ in the solution.

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Part (d)

1 point

Calculate the pH of the NH₃ solution before any HCl was added.

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Part (e)

1 point

Determine the pH of the mixture after 10.20 mL of HCl has been added. Justify your answer.

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Part (f)

2 points

(i) Explain why the pH at the equivalence point is less than 7. (ii) Calculate the pH at the equivalence point.

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Part (g)

1 point

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

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Part (h)

1 point

In aqueous solution, NH₃ reacts with water: NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq). Identify the Brønsted–Lowry acid in the forward reaction, and identify its conjugate base.

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Part (i)

1 point

Before the titration, the student rinsed the buret with distilled water but did not rinse it with the HCl solution, so a few drops of water diluted the HCl in the buret. Would the calculated [NH₃] be greater than, less than, or equal to the true value? Justify your answer.

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