A certain indicator, HA, has a Ka value of 6.3 × 10-8. The protonated form of the indicator is blue and the ionized form is red.
-What is the PKa of the indicator?
-What is the color of the indicator in a solution with pH=5?
pKa = -log Ka.
HA + H2O ==> H3o^+ + A^-
Ka = (H3O^+)(A^-)/(HA)
Then (A^-)/(HA) = Ka/(H3O^+).
Therefore, if pH = 5, then (H3O^+) = 1E-5. Substitute into Ka expression and (A^-)/(HA) = 6.3E-3.
When that ratio is = or > 10 you get the A^- color; when the ratio is = or < 1/10 you get the HA color.
To determine the pKa of the indicator, we can use the formula:
pKa = -log(Ka)
In this case, the Ka value is given as 6.3 × 10^-8. Plugging this value into the formula, we get:
pKa = -log(6.3 × 10^-8)
Using a calculator, you can calculate pKa to be approximately 7.2.
Now, let's determine the color of the indicator in a solution with pH=5. The pH scale ranges from 0 to 14, with pH 7 being neutral. A pH below 7 is acidic, while a pH greater than 7 is basic.
Since the pKa value of the indicator is 7.2, which is greater than the pH of 5, the indicator will be mostly in its protonated form at a pH of 5. The protonated form is blue, so the color of the indicator in this solution would be blue.
To answer these questions, we need to understand the concept of pKa and how it relates to the pH of a solution.
The pKa of an acid is a measure of its acidity, specifically the concentration of hydrogen ions when the acid is 50% ionized. It is a logarithmic scale, which means that each whole number difference in pKa corresponds to a tenfold difference in acidity.
To determine the pKa of the indicator, given its Ka value, we can use the formula:
pKa = -log10(Ka)
Let's plug in the given Ka value of the indicator:
pKa = -log10(6.3 × 10^-8)
Calculating this on a calculator or computer, we get:
pKa ≈ 7.20
So, the pKa of the indicator is approximately 7.20.
Now, let's determine the color of the indicator in a solution with pH = 5. The pH scale measures the concentration of hydrogen ions in a solution. The lower the pH value, the higher the concentration of hydrogen ions, and the more acidic the solution.
Since the pKa of the indicator is 7.20 and the pH of the solution is 5, we can compare these values to understand the color of the indicator.
When the pH of the solution is lower than the pKa of the indicator, the protonated form of the indicator predominates, and it appears blue. Conversely, when the pH is higher than the pKa, the ionized form predominates, and it appears red.
In this case, the pH of 5 is lower than the pKa of 7.20. Therefore, the protonated form of the indicator will predominate, and the color of the indicator in a solution with pH = 5 will be blue.