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AP Chemistry 8.4 Acid-Base Reactions and Buffers - Exam Style questions - FRQs- New Syllabus

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Question

A student is asked to prepare a buffer solution made with equimolar amounts of \( \mathrm{CH_3NH_2}(aq) \) and \( \mathrm{CH_3NH_3Cl}(s) \). The student uses \( 25.00\ \mathrm{mL} \) of \( 0.100\ \mathrm{M\ CH_3NH_2}(aq) \), which contains \( 0.00250\ \mathrm{mol} \) of \( \mathrm{CH_3NH_2} \), to make the buffer.
(a) Calculate the mass of \( \mathrm{CH_3NH_3Cl}(s) \) that contains \( 0.00250\ \mathrm{mol} \) of \( \mathrm{CH_3NH_3Cl} \).
The student has the following materials and equipment available.
• Distilled water      • Electronic balance      • \( 50\ \mathrm{mL} \) beaker      • Pipets
• \( 0.100\ \mathrm{M\ CH_3NH_2}(aq) \)      • Weighing paper      • \( 10.0\ \mathrm{mL} \) graduated cylinder      • pH meter
• Solid \( \mathrm{CH_3NH_3Cl} \)      • \( 50.00\ \mathrm{mL} \) buret      • Small spatula
(b) The following table contains a partial procedure for making the buffer solution. Fill in steps \( 1 \) and \( 4 \) to complete the procedure using only materials and equipment selected from the choices given. (Not all materials listed will be used. Assume that all appropriate safety measures are already in place.)
StepProcedure
\( 1 \) 
\( 2 \)Place the solid in the \( 50\ \mathrm{mL} \) beaker.
\( 3 \)Clean the buret and rinse with distilled water.
\( 4 \) 
\( 5 \)Use the buret to add \( 25.00\ \mathrm{mL} \) of \( 0.100\ \mathrm{M\ CH_3NH_2}(aq) \) to the beaker.
\( 6 \)Mix well.
\( 7 \)Check the pH with the pH meter.
The value of \( K_b \) for \( \mathrm{CH_3NH_2}(aq) \) is \( 4.4 \times 10^{-4} \), and the pH of the buffer the student prepared is \( 10.64 \).
(c) The student prepares a second buffer solution. The student uses \( 25.00\ \mathrm{mL} \) of \( 0.050\ \mathrm{M\ CH_3NH_2}(aq) \) instead of \( 25.00\ \mathrm{mL} \) of \( 0.100\ \mathrm{M\ CH_3NH_2}(aq) \), and half the mass of \( \mathrm{CH_3NH_3Cl}(s) \) that was used in the first buffer. Is the pH of the second buffer greater than, less than, or equal to the pH of the first buffer? Justify your answer.

Most-appropriate topic codes (AP Chemistry):

• Topic \( 4.5 \) — Stoichiometry (Part \( \mathrm{(a)} \))
• Topic \( 8.4 \) — Acid-Base Reactions and Buffers (Part \( \mathrm{(b)} \))
• Topic \( 8.9 \) — Henderson-Hasselbalch Equation (Part \( \mathrm{(c)} \))
▶️ Answer/Explanation

(a)
For the correct calculated value:
\( 0.00250\ \mathrm{mol\ CH_3NH_3Cl} \times \dfrac{67.52\ \mathrm{g}}{1\ \mathrm{mol}} = 0.169\ \mathrm{g} \)

A quick check: the problem says the buffer uses equimolar amounts, so the salt must also provide \( 0.00250\ \mathrm{mol} \), matching the moles of \( \mathrm{CH_3NH_2} \).

(b)
For a correct description of step \( 1 \):
Accept one of the following:

• Use the spatula, balance, and weighing paper to measure out exactly \( 0.169\ \mathrm{g} \) of \( \mathrm{CH_3NH_3Cl}(s) \).
• Use the balance to weigh out the mass of solid found in part \( \mathrm{(a)} \).

For a correct description of step \( 4 \):
Rinse the buret with a small amount of \( 0.100\ \mathrm{M\ CH_3NH_2}(aq) \), drain, and refill with \( 0.100\ \mathrm{M\ CH_3NH_2}(aq) \).

The buret should be rinsed with the solution it will contain so the concentration is not changed by leftover water.

(c)
For the correct answer and a valid justification:
Equal to. The ratio of weak acid to conjugate base is still \( 1:1 \).

In the second buffer, both the base \( \mathrm{CH_3NH_2} \) and its conjugate acid \( \mathrm{CH_3NH_3^+} \) are reduced by the same factor, so \( \dfrac{[\mathrm{CH_3NH_2}]}{[\mathrm{CH_3NH_3^+}]} \) stays the same.
Since buffer pH depends on this ratio, the pH remains unchanged.

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