Which of the following graphs shows how pH will vary when dilute HCl is added to 100 mL of dilute natural buffer solution with an initial pH of 7.0? (A) (B) (C) (D) - HSC - SSCE Chemistry - Question 15 - 2003 - Paper 1

When dilute HCl (a strong acid) is added to the buffer solution, the pH will initially decrease as the acid reacts with the buffer capacity until it is overwhelmed.

Graph A shows a gradual increase in pH, which is not a correct representation of the behavior of a buffer solution when an acid is added.

Graph B indicates a slight rise in pH, again not accurately depicting the behavior expected from a buffer solution with acid addition.

Graph C shows a rapid decrease in pH initially. This is partially correct as a buffer will show some drop in pH, but it should not remain low, indicating that this graph does not accurately represent the pH behavior under prolonged HCl addition.

Graph D depicts an initial sharp drop followed by a slight recovery, showcasing the buffer action at play. This is consistent with the expected behavior as some acid will be neutralized initially, but after a point, the addition will lead to drastic pH drop as the buffer capacity is surpassed.

Based on the analyses, Graph D is the most accurate representation of how the pH will vary when dilute HCl is added to the buffer solution.