Question

The pH of a solution of strong acid and weak base at the equivalence point is (a) 7 (b) more than 7 (c) less than 7 (d) none of the above

Short answer

The pH of a solution of strong acid and weak base at the equivalence point is (c) less than 7.

Step-by-step solution

Understand the nature of strong acids and weak bases

A strong acid is one that completely ionizes in water and contributes to lowering the pH by significantly increasing the \([H^{+}]\) concentration. A weak base, on the other hand, does not completely ionize in water and has a lower ability to accept protons and increase the \([OH^{-}]\) concentration.

Understand the nature of equivalence point in a titration

An equivalence point in a titration is reached when the moles of titrant (in this case, the strong acid) equal the moles of the analyte (in this case, the weak base). At the equivalence point, all of the weak base will react with the strong acid.

Determine the pH at the equivalence point

At the equivalence point, with the complete reaction of the weak base and the strong acid, the solution will be acidic due to the presence of the excess strong acid. Since pH is a measure of the acidity of a solution, it will fall under the acidic side of the pH scale, i.e., less than 7.

Key concepts

Acid-Base Titration

Acid-base titration is a laboratory procedure used to quantify the concentration of an unknown acid or base by reacting it with a known volume and concentration of a base or acid, respectively. The process of titration involves gradually adding one solution to the other while measuring how much is required to reach a chemical equivalence point. At this stage, the number of moles of hydrogen ions (([H^+)) from the acid equals the number of moles of hydroxide ions (([OH^-)) from the base, indicating that the reaction has fully occurred.

During the titration, indicators or pH meters are used to determine the equivalence point, which signals the completion of the reaction. In the context of a strong acid and weak base titration, the equivalence point is particularly important, as it influences the nature of the resulting solution and ultimately its pH.

For students aiming to understand this process, it is essential to comprehend the chemical reaction at the fundamental level, which involves tracking the ionic changes and considering the dissociation strengths of the reactants involved.

Strong Acids and Weak Bases

In a chemical context, the strength of an acid or a base refers to its ability to donate protons ([H^+) in the case of acids, or accept protons in the case of bases. Strong acids, such as hydrochloric acid (HCl) or sulfuric acid (H_2SO_4), dissociate completely in water, releasing a significant amount of [H^+ ions which contribute to the acidic nature of the solution. This complete ionization is what characterizes a strong acid.

Weak bases, on the other hand, like ammonia (NH_3), do not ionize completely and therefore generate fewer [OH^- ions in solution. This incomplete ionization results in a less pronounced increase in the solution's pH. It's important for learners to recognize these differences because they directly affect the pH level at the equivalence point during a titration. Understanding the intrinsic properties of strong acids and weak bases can provide valuable insights when predicting the resulting pH of a reaction.

pH Scale

The pH scale is a logarithmic measure that describes the acidity or alkalinity of a solution. It ranges typically from 0 to 14, with a pH of 7 being neutral. A pH less than 7 indicates an acidic solution, while a pH greater than 7 indicates a basic or alkaline solution.

To understand the pH scale from a chemical perspective, we consider the concentration of hydrogen ions ([H^+) in the solution. The pH scale is inversely related to this concentration: higher amounts of [H^+ ions result in a lower (more acidic) pH, while lower concentrations of [H^+ ions result in a higher (more alkaline) pH. In the context of acid-base titrations, monitoring the pH change throughout the process provides critical information about the reaction's progress and is pivotal in determining the equivalence point. Learners should be familiar with how to interpret pH values and the factors that affect these to have a robust understanding of acid-base chemistry.