Question

Compare the titration of a strong, monoprotic acid with a strong base to the titration of a weak, monoprotic acid with a strong base. Assume the strong and weak acid solutions initially have the same concentrations. Indicate whether the following statements are true or false. (a) More base is required to reach the equivalence point for the strong acid than the weak acid. (b) The pH at the beginning of the titration is lower for the weak acid than the strong acid. (c) The \(\mathrm{pH}\) at the equivalence point is 7 no matter which acid is titrated.

Short answer

(a) False: The same amount of base is required to reach the equivalence point for both the strong acid and the weak acid. (b) True: The pH at the beginning of the titration is lower for the strong acid than the weak acid. (c) False: For a strong acid-strong base titration, the pH at the equivalence point is 7, but for a weak acid-strong base titration, the pH at the equivalence point is higher than 7.

Step-by-step solution

Statement (a) Analysis

To determine whether more base is required to reach the equivalence point in the titration of the strong acid, we need to analyze the reaction between the strong acid and the strong base, as well as between the weak acid and the strong base. In both cases, the equivalence point is reached when the number of moles of acid and moles of base are equal. Since the initial concentrations of both acids are the same, and they are both monoprotic (donate one proton per molecule), the amount of base needed will be the same to reach the equivalence point.

Statement (a) Conclusion

Therefore, the statement (a) is False. The same amount of base is required to reach the equivalence point for both the strong acid and the weak acid.

Statement (b) Analysis

To compare the initial pH values of the strong and weak acids, we must understand how the pH values are determined and how strong and weak acids ionize in water. Strong acids such as HCl, \(\mathrm{H_2SO_4}\), and \(\mathrm{HNO_3}\) ionize completely in water, whereas weak acids such as acetic acid (CH3COOH) only ionize partially. As a result, the concentration of H+ ions in a strong acid solution will be higher than in a weak acid solution. The pH of a solution can be calculated using the following formula: \[pH = -\log[H^+]\] Using this formula, we can infer that a solution with a higher concentration of H+ ions will have a lower pH value.

Statement (b) Conclusion

Therefore, statement (b) is True. The pH at the beginning of the titration is lower for the strong acid than the weak acid, because the concentration of H+ ions in the strong acid solution is higher.

Statement (c) Analysis

The pH at the equivalence point depends on the products formed during the neutralization reaction between the acid and the base. For a strong acid-strong base titration, the product is a neutral salt dissolved in water. At the equivalence point, the concentrations of H+ and OH- ions are equal, forming water molecules and resulting in a pH value of 7. For a weak acid-strong base titration, the product is a basic salt dissolved in water. The basic anions formed from the weak acid (A-) can undergo hydrolysis, reacting with water to form OH- ions and increase the pH value. In this case, the pH at the equivalence point is higher than 7.

Statement (c) Conclusion

Therefore, statement (c) is False. The pH at the equivalence point is not 7 for all titrations. For a strong acid-strong base titration, the pH at the equivalence point is 7, but for a weak acid-strong base titration, the pH at the equivalence point is higher than 7.

Key concepts

Equivalence Point

The equivalence point in a titration is a crucial concept. It is the moment when the quantity of titrant added exactly neutralizes the amount of acid or base in the solution. At this stage, the moles of acid equal the moles of base.
The equivalence point does not depend on whether the acid is strong or weak. It is determined by stoichiometry alone. Since both strong and weak acids in the exercise are monoprotic and start with the same concentration, the same volume of base is required to reach the equivalence point in both cases.

• This means that for an equivalence point, the equation revolves around balancing the moles, rather than the strength of the acid.
• Therefore, more base isn’t necessary for one type of acid over the other; the same amount is needed as long as their concentrations and number of protons they can donate are equal.

Strong vs Weak Acids

Understanding the difference between strong and weak acids is fundamental in predicting their behavior in titrations.
Strong acids like hydrochloric acid ( HCl ), completely dissociate in water. This means they release more H^+ ions, making their initial pH lower. In contrast, weak acids like acetic acid ( CH_3COOH ) do not fully dissociate. This results in fewer H^+ ions in the solution and a higher initial pH .

• Due to complete ionization, solutions of strong acids have higher H^+ concentration, resulting in lower pH values initially.
• With partial ionization, weak acids contribute fewer H^+ ions, which means the starting pH is higher.

This characteristic greatly influences pH readings at different titration stages and determines the acid's initial pH values.

Titration pH

The pH of the solution during titration changes dynamically, and its behavior can be used to identify the reaction progress, particularly regarding the equivalence point. At the equivalence point, specifically, the nature of the acid (strong or weak) gives different results:
For a strong acid-strong base titration, both H^+ and OH^− ions neutralize completely at the equivalence point, often reaching a pH of 7. In contrast, with a weak acid-strong base titration, the equivalence point occurs at a pH greater than 7. This is due to the formation of a conjugate base from a weak acid, which has a tendency to react further, producing OH^− ions.

• Strong acid-strong base = Neutral pH , generally 7 at the equivalence point.
• Weak acid-strong base = Basic pH , greater than 7 at the equivalence point, as the conjugate base shifts the pH upwards.

This difference is important in calculating the end-point pH and provides valuable insights into the nature of the acid being titrated.