An acid-base indicator () has a . The acid form is red, and the basic form is blue. What is the pH c — Ionic Equilibrium Chemistry Question
Question
An acid-base indicator ($HIn$) has a $K_a = 1.0 \times 10^{-5}$. The acid form is red, and the basic form is blue. What is the pH change required to transition the color from 80% red to 80% blue?
Answer: A
💡 Solution & Explanation
$pH = pK_a + \log([In^-]/[HIn])$. At 80% red: $[In^-]/[HIn] = 20/80 = 1/4$. $pH_1 = 5 - \log 4$. At 80% blue: $[In^-]/[HIn] = 80/20 = 4/1$. $pH_2 = 5 + \log 4$. $\Delta pH = pH_2 - pH_1 = 2\log 4 = 2(0.60) = 1.20$.
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