1. The problem statement, all variables and given/known data I found this problem online for extra practice for my AP Chem class. https://www.quia.com/files/quia/users/jpugteach/APChem/AP-Chem-Chapter-19-Graphs.pdf The problem in question is number 4. I do not understand how to solve it and why the answer D is correct. 2. Relevant equations Henderson Hasselbalch Equation 3. The attempt at a solution Assuming I have an indicator of the form HIn <------> H+ + In- I know that my indicator will have a visible change when the ratio of In- to HIn will be 1/10. So I have pH = pKa + log ( [A-] / [HA] ) = -log ( 1.8 * 10 E -5 ) + log (1/10) = 3.744. So I put down that my answer was B as my calculated pH falls in that range. What am I doing wrong here? Thanks in advance for the help!