Short free-response question

Designing a pH 7.40 buffer

Unit 8
4 points
About 9 minutes

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

A shorter multipart question that sticks to one or two topics. Each part usually asks for one calculation, claim or explanation. On the exam: 4 questions (Questions 4–7), part of the 1-hour-45-minute free-response section; calculator allowed.

The question and its sources

A student needs to prepare a buffer solution with a pH of 7.40 at 25 °C. The student can choose one of the weak acids in Table 1, along with its sodium salt.

Table 1. Weak acids at 25 °C

Acid	Formula	Ka
Hydrofluoric acid	HF	6.8 × 10⁻⁴
Acetic acid	CH₃COOH	1.8 × 10⁻⁵
Hypochlorous acid	HOCl	3.0 × 10⁻⁸

Source: Standard values, rounded

Suggested time: 9 minutes

Your answers are saved in this browser as you type.

Part (a)

1 point

Which acid should the student choose to make the buffer? Justify your choice.

Your answer to part (a)

0 / 2,500 characters

Part (b)

1 point

Calculate the ratio [OCl⁻]/[HOCl] needed to give a buffer with a pH of 7.40.

Your answer to part (b)

0 / 2,500 characters

Part (c)

1 point

A small amount of HCl(aq) is added to the buffer. Write the balanced net ionic equation for the reaction that keeps the pH from changing significantly.

Your answer to part (c)

0 / 2,500 characters

Part (d)

1 point

Buffer X contains 0.40 M HOCl and 0.30 M NaOCl. Buffer Y contains 0.040 M HOCl and 0.030 M NaOCl. Which buffer has the greater capacity to resist a change in pH when HCl is added? Justify your answer.

Your answer to part (d)

0 / 2,500 characters

Checking scoring…

Scoring it yourself shows you the rubric, examples and a model answer. Try writing your answer first.