Phenolphthalein is a commonly used indicator that is colorless in the acidic 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? Hint given in feedback.

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? Hint given in feedback.

DrBob222 · Answer

I don't have the benefit of the "feedback" session but I would use 8.3 = 8.7 + log(b)/(a) solve for the ratio b/a.

carl · Answer

i did it i found 0.4 but wrong

DrBob222 · Answer

What was your feedback session?