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 \(\mathrm{pH}\) at the beginning of the titration is lower for the weak acid than the strong acid. \((\mathbf{c})\) The pH at the equivalence point is 7 no matter which acid is titrated.

Short answer

(a) False, (b) False, (c) False.

Step-by-step solution

Analyzing Equivalence Point Volume

For both strong and weak monoprotic acids, the equivalence point occurs when the moles of acid equal the moles of base added. Since both acid solutions start with the same concentration and are monoprotic, the amount of base needed to reach the equivalence point is the same for both acids. Hence, statement (a) is False.

Comparing Initial pH Values

A strong acid completely dissociates in water, resulting in a lower initial pH, while a weak acid only partially dissociates, resulting in a higher initial pH for the same concentration. Thus, the statement that the initial pH is lower for the weak acid is False, making statement (b) False.

Evaluating pH at the Equivalence Point

For titration of a strong acid with a strong base, the equivalence point pH is 7 because the neutralization forms a neutral salt and water. For a weak acid with a strong base, the pH at the equivalence point is greater than 7 due to the formation of a weak base. Hence, statement (c) is False.

Key concepts

Equivalence Point

During a titration, the equivalence point is a pivotal moment where the amount of titrant added is just enough to completely neutralize the analyte solution. In simpler terms, it's the point where the number of moles of acid equals the number of moles of base. This is critical because it marks the complete reaction between the titrant and the substance being analyzed.

For both strong and weak monoprotic acids, reaching the equivalence point does not depend on the strength of the acid. Since both types of acids can require the exact same amount of base to achieve this state, the volume of base needed remains consistent, assuming equal initial concentrations. This is because each acid molecule is neutralized by one base molecule.

• The equivalence point volume is the same for strong and weak acids if they have the same concentration.
• The strength of the acid does not affect the volume of base required; instead, it affects the pH at the equivalence point.

This understanding is critical for interpreting the titration process and dispels the common misconception that more base is needed for strong acids.

pH Value

The concept of pH is central to understanding titration curves and acid-base reactions. The pH scale measures how acidic or basic a solution is, with lower values indicating strong acidity, and higher values indicating basicity. At the start of a titration, the initial pH reflects the degree to which an acid dissociates in a solution.

For strong acids, which dissociate completely in water, the initial pH is lower. This is because the strong acid releases more hydrogen ions (H extsuperscript{+}) into the solution. In contrast, weak acids only partially dissociate, resulting in a higher initial pH even if the concentration is the same.

• Initial pH is distinctly lower for strong acids compared to weak acids at the same concentration.
• Understanding initial pH helps to anticipate the progression of the titration curve.

Differences in initial pH are essential for predicting how the solution will behave when the base is added and underscore the importance of acid strength in titration scenarios.

Strong Acid

A strong acid is characterized by its complete dissociation in water, meaning that all its molecules release hydrogen ions. This trait leads to a low pH, typically below 3 for common strong acids like hydrochloric acid (HCl). When titrating a strong acid with a strong base like sodium hydroxide (NaOH), the sharp change in pH near the equivalence point is notable.

Upon reaching the equivalence point, the pH in such titrations is around 7. This is because a neutral salt and water are produced, which are balanced in terms of acidity and basicity. The straightforward reaction profile of strong acids offers a clear demarcation at the equivalence point.

• Complete dissociation results in a low initial pH.
• Equivalence point with strong bases remains at pH 7, indicating neutralization.

The clarity of changes during titration makes strong acids ideal for educational settings where observable results foster understanding.

Weak Acid

Unlike strong acids, weak acids do not completely dissociate in solution, meaning only a portion of their molecules release hydrogen ions. This partial dissociation results in a higher initial pH compared to strong acids of the same concentration. Common weak acids include acetic acid (CH extsubscript{3}COOH).

When weak acids undergo titration with strong bases, the pH at the equivalence point exceeds 7. This is due to the formation of a weak conjugate base, which makes the resulting solution slightly basic. The titration curve for weak acids has a less steep slope near the equivalence point compared to strong acids.

• Weak acids have higher initial pH due to partial dissociation.
• They result in a basic pH at the equivalence point when titrated with strong bases.

Understanding the behavior of weak acids during titration is crucial for anticipating how pH levels change throughout the process and for interpreting titration curves effectively.