Indicators are compounds that change color with pH. Most indicators are actually
ID: 908614 • 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 pKa near the midpoint of that range. For example, methyl red (pKa=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=pKa=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.
HX ? H+ + X? color 1 color 2Explanation / Answer
Part A:
Indicators usually change color over a range of 1 to 3 pH units, with the pKa near the midpoint of that range.
So, pKa = 5.2 + 6.8 / 2 = 6
Ka = 10^(-6) = 1E-6
PartB: Given, Ka = 2E-5
So, pKa = 4.7
So, it will be green when pH = pKa = 4.7