Indicators are compounds that change color with pH. Most indicators are actually
ID: 1016132 • Letter: I
Question
Indicators are compounds that change color with pH. Most indicators are actually weak acids that are one color when protonated and another color when ionized: HX rightarrow H^+ + X^- color 1 color 2 Indicators usually change color over a range of 1 to 3 pH units, with the pK_a near the midpoint of that range. For example, methyl red (pK_a = 5) appears red in its acidic form (HX) and yellow in its basic form (X^-). This color change occurs gradually between pH = 4 and pH = 6. At any pH below 4, methyl red will appear red because there is significantly more HX than X^- present. At pH = pK_a = 5, methyl red will be orange because there will be equal amounts of HX and X^-. At any pH above 6, methyl red will appear yellow because there is significantly more X^- than HX present. Methyl violet is an indicator that changes color over a range from pH = 0.0 to pH = 1.6. What is K_a of methyl violet? Express your answer to one significant figure. K_a = 0.1 Phenolphthalein, K_a = 7.0 times 10^-10, is colorless in its protonated form (HX) and fuchsia in its ionized form (X^-). At what pH will phenolphthalein be a perfect pink color? Express your answer to one decimal place. pH = This question will be shown after you complete previous question(s).Explanation / Answer
part a)
pH = 1.6 -0.0 /2 = 0.8
so from pH= - log[H+]
[H+] = 0.2
as the [HX] = [X-]
Ka = [H+] = 0.2
partb)
from equation , pH= pKa + log[x-]/[HX]
where [X-] = [HX]
so pH = - log( 7.0 x 10^-10)
pH = 9.16