To determine which graph illustrates the variation of the pH of a strong acid solution as a strong base is titrated against it, we need to understand the behavior of pH during a titration process. ### Step-by-Step Explanation: 1. **Understanding Strong Acid and Strong Base**: - A strong acid (like hydrochloric acid, HCl) completely dissociates in water, meaning it releases all of its hydrogen ions (H⁺) into the solution. - A strong base (like sodium hydroxide, NaOH) also completely dissociates, releasing hydroxide ions (OH⁻). 2. **Titration Process**: - During the titration of a strong acid with a strong base, the pH of the solution will change as the base is added. - Initially, when no base is added, the pH of the strong acid solution is low (typically below 3, depending on the concentration of the acid). - As the strong base is added, it reacts with the hydrogen ions from the acid to form water, gradually increasing the pH. 3. **Key Phases of the Titration**: - **Initial Phase**: The pH starts low due to the strong acid. - **Buffer Region**: As base is added, the pH rises slowly at first because the acid is still in excess. - **Equivalence Point**: At this point, the amount of base added is stoichiometrically equivalent to the amount of acid present. For a strong acid-strong base titration, the pH at the equivalence point is typically around 7, as the resulting solution contains only water and a neutral salt. - **Post-Equivalence**: After the equivalence point, the pH rises sharply because any additional base will significantly increase the concentration of hydroxide ions in the solution. 4. **Graph Characteristics**: - The graph should start at a low pH, show a gradual increase as base is added, have a steep rise around the equivalence point, and then level off at a higher pH as more base is added. - The steep rise is characteristic of strong acid-strong base titrations, where the pH changes rapidly around the equivalence point. ### Analyzing the Options: - **Option A (I)**: If this graph shows a low starting pH, a gradual increase, a sharp rise at the equivalence point, and then levels off, it is likely the correct choice. - **Option B (II)**: If this graph shows a gradual increase without a sharp rise, it may represent a weak acid-strong base titration, which does not fit our scenario. - **Option C (III)**: If this graph starts high or has an unusual shape, it may not represent a strong acid-strong base titration. - **Option D (IV)**: If this graph shows a flat line or an incorrect pH range, it is also not suitable. ### Conclusion: The correct option is **A** because it accurately represents the expected pH changes during the titration of a strong acid with a strong base, showing the initial low pH, gradual increase, sharp rise at the equivalence point, and leveling off afterward. ### Revision Summary: - Strong acids completely dissociate in solution, leading to low initial pH. - Titration with a strong base results in a gradual pH increase, a sharp rise at the equivalence point, and a leveling off. - The equivalence point for strong acid-strong base titrations is typically around pH 7. - The correct graph will show these characteristics: low starting pH, gradual increase, steep rise, and leveling off.