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Difficulty: EasyRelative Strength and Ionization of Acids and Bases

Two aqueous solutions, one of ethanoic acid (CH3COOH\text{CH}_3\text{COOH}) and the other of hydrochloric acid (HCl\text{HCl}), have the exact same concentration of 0.10 mol dm−30.10\text{ mol dm}^{-3}. Which of the following statements correctly accounts for why hydrochloric acid is classified as a stronger acid than ethanoic acid?

  1. Hydrochloric acid completely ionizes in aqueous solution, whereas ethanoic acid only partially ionizes.Answer
  2. B
    Hydrochloric acid has a higher molar concentration of acid molecules in the solution than ethanoic acid.
  3. C
    Ethanoic acid undergoes complete dissociation in water while hydrochloric acid forms a reversible equilibrium.
  4. D
    Ethanoic acid produces a higher hydrogen ion concentration because it contains four hydrogen atoms per molecule.

Answer

Hydrochloric acid completely ionizes in aqueous solution, whereas ethanoic acid only partially ionizes.
The strength of an acid is defined by its degree of ionization in aqueous solution. Hydrochloric acid (HCl\text{HCl}) is a strong acid because it ionizes completely in water to yield hydrogen ions. In contrast, ethanoic acid (CH3COOH\text{CH}_3\text{COOH}) is a weak acid that ionizes only partially, setting up a dynamic equilibrium mixture of molecules and ions.

Step-by-Step Solution

1
Define acid strength in terms of degree of ionization.
Acid strength is determined by the extent to which an acid dissociates into hydrogen ions (H+\text{H}^+) in water, independent of solution concentration.
Strong acids dissociate completely, whereas weak acids dissociate only partially.
2
Compare the ionization behavior of hydrochloric acid and ethanoic acid.
Hydrochloric acid (HCl\text{HCl}) is a strong monobasic acid that ionizes virtually 100%100\% in water. Ethanoic acid (CH3COOH\text{CH}_3\text{COOH}) is a weak monobasic acid that ionizes only slightly, establishing a dynamic equilibrium.
Because both solutions have equal molar concentrations, the completely ionized strong acid yields a substantially greater concentration of H+\text{H}^+ ions.

Key Concept

Relative Strength and Ionization of Acids and Bases
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Relative Strength and Ionization of Acids and Bases

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