Indicators are compounds that change color with pH. Most indicators are actually
ID: 1015810 • 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: 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. (Figure 1) 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. 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. This question will be shown after you complete previous question(s).Explanation / Answer
pKa is generally at the midpoint of the range in which it changes colour.
Methyl violet colour changes in range of pH 0 - 1.6.
pKa = 1.6/2 = 0.8
Ka = 10^-0.8 = 0.158
Phenolphathalein is pink at neutral point that is X- = HX
At pKa this happpens so pH = pKa = -log(7 * 10^-10) = 9.15