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

STEPS:

1. Identify the chemical equation and stoichiometry of the reaction:
All four haloacetic acids—fluoroacetic, chloroacetic, bromoacetic, and iodoacetic acid—are monoprotic weak acids. When titrated with sodium hydroxide, they react according to a $1:1$ stoichiometric ratio where one mole of hydroxide ions reacts with one mole of the monoprotic acid to form water and the conjugate base:
$$\text{HA}(aq) + \text{OH}^-(aq) \rightarrow \text{A}^-(aq) + \text{H}_2\text{O}(l)$$

2. Define the equivalence point of a titration:
The equivalence point in an acid-base titration is the point at which the number of moles of added titrant ($\text{NaOH}$) is stoichiometrically equal to the number of moles of acid analyte originally present in the sample. For a monoprotic acid titrated with a monoprotic strong base, this occurs when the moles of $\text{OH}^-$ added equals the moles of $\text{HA}$ originally present in the flask.

3. Calculate the moles of analyte present in each titration flask:
The problem specifies that the student titrates $10.0\text{ mL}$ samples of $1.0\text{ M}$ solutions of each of the haloacetic acids. Because the initial concentrations and volumes are identical across all trials, each sample contains the exact same number of moles of acid analyte:
$$\text{Moles of Acid} = \text{Molarity} \times \text{Volume} = 1.0\text{ mol/L} \times 0.0100\text{ L} = \mathbf{0.010\text{ mol of Acid}}$$
Thus, equimolar amounts of each acid are being analyzed.

4. Determine the required volume of titrant:
Because every sample contains exactly $0.010\text{ mol}$ of a monoprotic acid, each titration requires exactly $0.010\text{ mol}$ of $\text{NaOH}$ to reach the equivalence point. Since the exact same standard $\text{NaOH}$ solution is used as the titrant for all runs, the volume of $\text{NaOH}$ solution needed to supply this amount of base is identical for all four acids, identifying Option C as the correct choice.

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WHY_OTHERS_WRONG:

• Options A and B are incorrect: These options suggest that fluoroacetic acid or iodoacetic acid would require a smaller volume of base to reach the equivalence point. Students often make this mistake by confusing acid strength (indicated by $K_a$ or percent ionization) with the total amount of neutralizing base needed. While fluoroacetic acid is the strongest of these weak acids (highest $K_a$) and iodoacetic acid is the weakest (lowest $K_a$), titration equivalence points depend entirely on the total moles of acidic protons present, not on the degree to which those acids dissociate in water before titration.
• Option D is incorrect: This option claims that because the haloacetic acids are weak, they will not reach an equivalence point. This is a fundamental misconception; weak acids absolutely reach a stoichiometric equivalence point when titrated with a strong base because the strong base drives the neutralization reaction to completion.

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🌡️ Question 29 is the next in this exam sequence and asks about the autoionization of water at an elevated temperature of 40°C. Would you like to walk through how to calculate the pH of pure water under these conditions?