AP Chemistry 8.9: Buffer pH

Practice AP Chemistry questions on calculating buffer pH or conjugate-acid/base ratios with pKa and evaluating whether a buffer suits a target pH.

Exam points

Explain how redox reactions transfer electrons and change oxidation states in chemical systems.
Apply oxidation numbers and half-reactions to balance equations and analyze electrochemical cells.

[Maximum number: 1]

Answer the following questions about ascorbic acid (vitamin C).

Ascorbic acid, HAsc (a q), acts as a weak acid, as shown in the equation.

\mathrm{HAsc}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftarrows \mathrm{H}_{3} \mathrm{O}^{+}(a q)+\mathrm{Asc}^{-}(a q)

The following titration curve was produced when a 10.0 mL sample of HAsc (a q) was titrated

using $0.0550 M \mathrm{NaOH}(a q)$ .

Figure for Question 8.9.A—Identify the pH of a buffer solution based on the identity and concentrations of the conjugate acid-base pair… question 1 — AP Chemistry

What is the value of the ratio $\frac{\left[\mathrm{Asc}^{-}\right]}{[\mathrm{HAsc}]}$ when the pH of the solution is 4.7?

(iii) For the correct ratio, consistent with part B (ii):

Using the Henderson-Hasselbalch equation:

$\mathrm{pH}=\mathrm{p} K_{a}+\log \left(\frac{\left[\mathrm{Asc}^{-}\right]}{[\mathrm{HAsc}]}\right)$ $4.7=4.1+\log \left(\frac{\left[\mathrm{Asc}^{-}\right]}{[\mathrm{HAsc}]}\right)$ $\frac{\left[\mathrm{Asc}^{-}\right]}{[\mathrm{HAsc}]}=10^{0.6}=4.0$

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8.9.A—Identify the pH of a buffer solution based on the identity and concentrations of the conjugate acid-base pair… question 1