the student stopped the first careful titration when a deep pink color was obtained in the flask. On the second trial, the student stopped when a light pink color was obtained. On the third trial, the pink color obtained was very faint and noticeable only when the flask was held in front of a white piece of paper. Which of these trials will give the most accurate calculation of the acid concentration? Explain your answer.

What were you titrating? In some cases I might answer the light pink. In others I might answer the faint pink visible only when held against a white background. In general, however, the last one usually is taken as the end point.