Questions 26-28 refer to the following information. The structure of haloacetic acids, XCH2COOH (whe — Acids and Bases Chemistry Question

STEPS:

1. Retrieve the acid dissociation constant ($K_a$) or $pK_a$ values from the table:
According to the provided data table for haloacetic acids:
* Fluoroacetic acid: $K_a = 2.57 \times 10^{-3}$ (or $pK_a = 2.59$).
* Chloroacetic acid: $K_a = 1.35 \times 10^{-3}$ (or $pK_a = 2.87$).

2. Relate $K_a$ and $pK_a$ to acid strength:
The acid dissociation constant ($K_a$) quantitatively measures the strength of an acid in solution.
* A larger $K_a$ value (which corresponds to a smaller $pK_a$ value) indicates a stronger weak acid because the equilibrium lies further to the product (ionized) side.
* Comparing the two, fluoroacetic acid has a larger $K_a$ ($2.57 \times 10^{-3} > 1.35 \times 10^{-3}$) and a lower $pK_a$ ($2.59 < 2.87$), meaning fluoroacetic acid is a stronger acid than chloroacetic acid.

3. Connect acid strength to percent ionization:
Percent ionization is defined as the ratio of the equilibrium concentration of ionized acid ($[\text{H}^+]_{eq}$) to the initial concentration of the weak acid ($[\text{HA}]_0$), multiplied by 100%:
$$\text{Percent Ionization} = \frac{[\text{H}^+]}{[\text{HA}]} \times 100\%$$
For weak monoprotic acids at equal initial concentrations (both are $0.10\text{ M}$ in this scenario), the stronger acid will dissociate to a greater extent, yielding a higher concentration of hydronium ions ($\text{H}^+$) at equilibrium. Consequently, the stronger acid will always exhibit a higher percent ionization.

4. Compare the percent ionization of the two acids:
Since fluoroacetic acid is the stronger of the two acids, it ionizes to a greater extent than chloroacetic acid at equal concentrations. Therefore, the percent ionization of chloroacetic acid must be less than the percent ionization of fluoroacetic acid. This directly aligns with Option B.

*

WHY_OTHERS_WRONG:

• Option A is incorrect: This option falsely claims that chloroacetic acid has a greater percent ionization than fluoroacetic acid. This is incorrect because chloroacetic acid is a weaker acid (smaller $K_a$) and thus dissociates to a lesser extent at equal concentrations.
• Option C is incorrect: The initial concentrations of both acids are explicitly specified in the prompt as equal ($0.10\text{ M}$). Even if the concentration values were omitted, as long as they are known to be equal, their percent ionizations can be compared directly using their relative $K_a$ values.
• Option D is incorrect: The pH of the solution is a dependent variable that results from the ionization of the acid, not an independent piece of information required to compare their baseline strengths. Knowing the relative $K_a$ values and initial concentrations is entirely sufficient to determine which acid has a greater percent ionization.