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Chapter 17: Problem 16

A student wishes to prepare a buffer solution at \(\mathrm{pH}=\) \(8.60 .\) Which of these weak acids should she choose and why: HA \(\left(K_{\mathrm{a}}=2.7 \times 10^{-3}\right),\) HB \(\left(K_{\mathrm{a}}=4.4 \times\right.\) \(10^{-6}\) ), or HC \(\left(K_{\mathrm{a}}=2.6 \times 10^{-9}\right) ?\)

Short Answer

Expert verified
The student should choose weak acid HC to prepare the buffer solution, as its pKa value is closest to the desired pH of 8.60.

Step by step solution

01

Convert Ka values to pKa values

To convert Ka to pKa, use the formula \( \text{pKa} = -\log(K_a) \). Thus, the pKa for HA is \( -\log(2.7 \times 10^{-3}) = 2.57 \), for HB is \( -\log(4.4 \times 10^{-6}) = 5.36 \), and for HC is \( -\log(2.6 \times 10^{-9}) = 8.59.
02

Compare pKa values with the desired pH

The pKa for the acid should be close to the desired pH for the buffer solution. In this case, the desired pH is 8.60. So, by comparing the calculated pKa values with the desired pH, it can be seen that the pKa of HC (\(8.59\)) is closest to the desired pH.
03

Select the appropriate weak acid

The student should select the weak acid with a pKa value closest to the desired pH. In this case, HC is the appropriate weak acid to be chosen to prepare the buffer solution.

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Most popular questions from this chapter

What is the \(\mathrm{pH}\) of the buffer \(0.10 \mathrm{M} \mathrm{Na}_{2} \mathrm{HPO}_{4} / 0.15 \mathrm{M}\) \(\mathrm{KH}_{2} \mathrm{PO}_{4} ?\)

A 1.0 -L saturated silver carbonate solution at \(5^{\circ} \mathrm{C}\) is treated with enough hydrochloric acid to decompose the compound. The carbon dioxide generated is collected in a \(19-\mathrm{mL}\) vial and exerts a pressure of \(114 \mathrm{mmHg}\) at \(25^{\circ} \mathrm{C}\). What is the \(K_{\mathrm{sp}}\) of \(\mathrm{Ag}_{2} \mathrm{CO}_{3}\) at \(5^{\circ} \mathrm{C} ?\)

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Define \(\mathrm{p} K_{\mathrm{a}}\) for a weak acid and explain the relationship between the value of the \(\mathrm{p} K_{\mathrm{a}}\) and the strength of the acid. Do the same for \(\mathrm{p} K_{\mathrm{b}}\) and a weak base.

The amount of indicator used in an acid-base titration must be small. Why?
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