Which solution will be more acidic; that is, which will have...

Which solution will be more acidic; that is, which will have a lower pH? (a) $0.10 M \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}$ (phenol) or $0.10 \mathrm{M} \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}$ (ethanol) (b) $0.10 M \mathrm{NH}_{3}$ or $0.10 \mathrm{M} \mathrm{NH}_{4} \mathrm{Cl}$ (c) $0.10 M \mathrm{NaCl}$ or $0.10 \mathrm{M} \mathrm{NH}_{4} \mathrm{Cl}$ (d) $0.10 M \mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH}) \mathrm{COOH}$ (lactic acid) or $0.10 \mathrm{M}$ $\mathrm{CH}_{3} \mathrm{COOH}$ (e) $\left.0.10 M \text { ascorbic acid (vitamin } C, p K_{n}=4.1\right)$ or $0.10 M$ acetic acid

Which solution will be more acidic; that is, which will have a lower pH?
(a) $0.10 M \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}$ (phenol) or $0.10 \mathrm{M} \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}$
(ethanol)
(b) $0.10 M \mathrm{NH}_{3}$ or $0.10 \mathrm{M} \mathrm{NH}_{4} \mathrm{Cl}$
(c) $0.10 M \mathrm{NaCl}$ or $0.10 \mathrm{M} \mathrm{NH}_{4} \mathrm{Cl}$
(d) $0.10 M \mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH}) \mathrm{COOH}$ (lactic acid) or $0.10 \mathrm{M}$
$\mathrm{CH}_{3} \mathrm{COOH}$
(e) $\left.0.10 M \text { ascorbic acid (vitamin } C, p K_{n}=4.1\right)$ or $0.10 M$ acetic acid

Key Concepts

Acid Dissociation Constant (Ka) and pKa

Acid dissociation constant (Ka) is a quantitative measure of the strength of an acid in solution, and its negative logarithm (pKa) provides a convenient way to compare acid strengths. A lower pKa value indicates a stronger acid that dissociates more readily, releasing hydrogen ions into the solution. This concept is crucial for determining which solution will be more acidic because comparing the pKa values of different acids allows one to predict the extent of ionization and, therefore, the resulting pH.

Base Ionization and pKb

For bases, the ionization constant (Kb) and its negative logarithm (pKb) are used to measure the strength of a base. A lower pKb indicates a stronger base. In acid-base comparisons involving species that can donate or accept protons, knowing the pKb of a weak base and recognizing its conjugate acid behavior (e.g., ammonium from ammonia) is essential. This helps in predicting whether the solution will lean towards acidic or basic pH values.

Acid-Base Equilibrium

Acid-base equilibrium refers to the state in which the rate of proton donation by an acid equals the rate of proton acceptance by a base, resulting in a balance between the undissociated acid and its ions. Understanding these equilibria, especially for weak acids and bases, is key to predicting the final pH of a solution. The extent to which acids or bases dissociate determines the concentration of hydrogen or hydroxide ions in solution, which directly influences the pH.

Salt Hydrolysis

Salt hydrolysis involves the reaction of salt ions with water, which can lead to either an acidic or basic solution. The ion derived from a weak acid or a weak base can combine with water to either release hydrogen ions or hydroxide ions. Recognizing which salts undergo hydrolysis—and whether the resulting solution will be acidic, basic, or neutral—is crucial when comparing the pH of solutions containing different salts.

Structural Effects on Acidity

The molecular structure of a compound, including the presence of electronegative atoms, resonance stabilization, and inductive effects, plays a significant role in determining acid strength. Structural factors affect how easily an acid can donate a proton, thereby influencing its pKa. This concept is particularly important when comparing molecules like phenol versus ethanol or lactic acid versus acetic acid, where differences in structure lead to notable differences in acid strength and pH.

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