Phenolphthalein is a commonly used indicator that is colorless in the acidic for
ID: 605157 • Letter: P
Question
Phenolphthalein is a commonly used indicator that is colorless in the acidic form (pH < 8.3) and pink in the basic form (pH > 10.0). It is a weak acid with a pKa of 8.7. What is the ratio of the conjugate base concentration to the acid concentration, that is, [B- ]/[HB] when the indicator switches to the acidic color? 1. Use the result to the above question to determine what percent is in the acidic form when the acid color is first apparent? 2. What is [B- ]/[HB] when the indicator switches to the basic color? 3. Use the result to the above question to determine what percent is in the basic form when the base color is first apparent?Explanation / Answer
a) pH = pKa + log [B-]/[HB] 8.0 = 8.7 + log [B-]/[HB] [B-]/[HB] = 0.200 b) 10 = 8.7 + log [B-]/[HB] [B-]/[HB] = 20 c) Since [B-]/[HB] = 20 then 20 parts of the indicator are in the B- form and 1 part is in the HB form. So, 20/21 X 100 = 95% of the molecules have to be in the B- form to show the base color.