Two 25.0-mL samples, one 0.100 M HCl and the other 0.100 M HF, are titrated with 0.200 M KOH. a. What is the volume of added base at the equivalence point for each titration? b. Is the pH at the equivalence point for each titration acidic, basic, or neutral? c. Which titration curve has the lower initial pH? d. Sketch each titration curve.

Understanding the equivalence point in a titration is critical. Simply put, it's the moment when the amount of added titrant (in this case, KOH) equals the number of substance being titrated (HCl or HF). At this juncture, the reaction is complete. For the titration of a strong acid like HCl with KOH, the equivalence point will occur when the volume of 0.200 M KOH equals the number of moles of 0.100 M HCl. Since both are in a 1:1 molar ratio, you'd need half the volume of 0.200 M KOH, which is 12.5 mL, to reach the equivalence point with 25 mL of 0.100 M HCl.

With HF, a weak acid, the same calculation applies because it still reacts in a 1:1 ratio with KOH. However, the nature of the acid influences the pH at the equivalence point, which won’t be neutral due to the presence of the fluoride ion, F-, which slightly affects the hydrogen ion concentration, leaning towards basic pH values.

To calculate pH, one must understand the level of the hydrogen ion concentration in the solution. For strong acids like HCl, this computation is straightforward since they fully dissociate in water, making the hydrogen ion concentration equal to the acid concentration. Hence for 0.100 M HCl, the pH would be 1. For weak acids like HF, the calculation is more complex due to partial dissociation. The presence of the conjugate base (F-) at the equivalence point of weak acids affects the pH, making the solution basic. For example, the fluoride ion somewhat grabs hydrogen ions from water, leading to a pH greater than 7.

Determining the pH at various points during the titration process requires understanding of buffer solutions and hydrolysis, particularly in the buffer region and beyond the equivalence point.

Acids are deemed strong or weak based on their ability to dissociate in water. Strong acids, like HCl, ionize completely in aqueous solution, releasing all of their hydrogen ions, hence they have a lower pH. Weak acids, such as HF, only partially dissociate, donating fewer hydrogen ions into the solution, and resulting in a higher initial pH. This distinction plays a key role in predicting the behavior of acids during a titration and their corresponding titration curves.

When doing a titration, it's essential to note that strong acids will react with strong bases to form neutral water and a salt, while weak acids will typically lead to the formation of basic solutions at the equivalence point due to the weakly basic nature of the conjugate base.

A titration curve is a graphic representation of pH versus the volume of titrant added. The shape of the curve provides insights into the acid and base properties in the reaction. For strong acids titrated with strong bases, expect the curve to start with a low pH and increase sharply near the equivalence point, leveling off at pH 7. In this scenario, HCl would present such a curve.

For weak acids like HF, the curve starts at a higher pH and increases more gradually, with a buffer region where the pH doesn't change much even when more base is added. At the equivalence point for weak acids, there's a less steep but still noticeable rise, leveling off at a pH above 7 because of the conjugate base's tendency to increase the solution's pH. These curves not only assist in identifying equivalence points but also tell us about the strength of the acids and bases involved in the reaction.