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) $0.10 \mathrm{M} \mathrm{CH}_{3} \mathrm{COOH}$ or $0.10 \mathrm{M} \mathrm{HCl}$ (b) $0.10 M \mathrm{CH}_{3} \mathrm{COOH}$ or $0.10 \mathrm{M} \mathrm{H}_{3} \mathrm{PO}_{4}$ (c) $0.010 M \mathrm{H}_{2} \mathrm{CO}_{3}$ or $0.010 \mathrm{M} \mathrm{NaHCO}_{3}$ (d) $0.10 M \mathrm{NaH}_{2} \mathrm{PO}_{4}$ or $0.10 \mathrm{M} \mathrm{Na}_{2} \mathrm{HPO}_{4}$ (e) $0.10 M$ aspirin $\left(\mathrm{p} K_{\mathrm{a}}=3.47\right)$ or $0.10 \mathrm{M}$ acetic acid

Which solution will be more acidic; that is, which will have a lower pH?
(a) $0.10 \mathrm{M} \mathrm{CH}_{3} \mathrm{COOH}$ or $0.10 \mathrm{M} \mathrm{HCl}$
(b) $0.10 M \mathrm{CH}_{3} \mathrm{COOH}$ or $0.10 \mathrm{M} \mathrm{H}_{3} \mathrm{PO}_{4}$
(c) $0.010 M \mathrm{H}_{2} \mathrm{CO}_{3}$ or $0.010 \mathrm{M} \mathrm{NaHCO}_{3}$
(d) $0.10 M \mathrm{NaH}_{2} \mathrm{PO}_{4}$ or $0.10 \mathrm{M} \mathrm{Na}_{2} \mathrm{HPO}_{4}$
(e) $0.10 M$ aspirin $\left(\mathrm{p} K_{\mathrm{a}}=3.47\right)$ or $0.10 \mathrm{M}$ acetic acid

Key Concepts

pH is the measure of the hydrogen ion concentration in a solution and is an indicator of its acidity or basicity. A lower pH corresponds to a higher concentration of H+ ions and thus a more acidic solution. The pH of a solution is influenced by both the concentration and the ionization properties of the acids present.

Polyprotic Acids

Polyprotic acids, such as those that can donate more than one proton, have multiple dissociation equilibria with different Ka values for each proton release. This makes the pH determination more complex, as each step contributes differently to the overall acidity of the solution.

Buffer Systems

Buffer systems involve a weak acid and its conjugate base (or a weak base and its conjugate acid) and work to resist changes in pH when small amounts of acid or base are added. Understanding buffers is important when comparing solutions that contain salts derived from weak acids, as their pH is determined by the ratio of these species.

Acid Dissociation Constant (Ka) and pKa

The acid dissociation constant (Ka) quantifies the extent to which an acid donates its proton, while the pKa, which is the negative logarithm of Ka, provides a more convenient measure of acid strength. A lower pKa value indicates a stronger acid that is more likely to dissociate and produce H+ ions in solution, leading to a lower pH.

Acid Strength

Acid strength refers to the tendency of an acid to donate protons (H+) when dissolved in water. Strong acids completely ionize and produce a high concentration of H+ ions, thus lowering the pH substantially, whereas weak acids only partially ionize and result in a higher pH compared to strong acids at the same concentration.

Strong Acids versus Weak Acids

This concept distinguishes between acids that fully dissociate in aqueous solutions (strong acids) and those that only partially dissociate (weak acids). The degree of dissociation directly affects the concentration of hydrogen ions in the solution and therefore its pH.

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