The indicator bromocresol green (HIn) can be in two different forms: As HIn the color of the indicator is yellow, an In the color of the indicator is blue, and an equilmolar mixture of the two species is green. The Ka of the indicator is 2.5 x 10-5. What is the color of the indicator in a solution of pH= 3.8?
My blind guess would be HIn, yellow since that's an acid, but there may be more to the question that I'm not factoring in.
To determine the color of the indicator in a solution of pH 3.8, we need to compare the pH of the solution to the pKa of the indicator.
In this case, the indicator is bromocresol green, which can exist in two forms: HIn (acidic form) and In- (basic form). The pH at which half of the indicator is in the acidic form and half is in the basic form is called the pKa of the indicator.
The pKa of an indicator is related to its Ka (acid dissociation constant) by the equation pKa = -log(Ka). In this case, the given Ka is 2.5 x 10^-5. Therefore, we can calculate the pKa as follows:
pKa = -log(2.5 x 10^-5) = 4.6
Now, we compare the pH of the solution (pH= 3.8) to the pKa of the indicator (pKa = 4.6). If the pH is lower than the pKa, the acidic form (HIn) will predominate, and the indicator will appear yellow. If the pH is higher than the pKa, the basic form (In-) will predominate, and the indicator will appear blue. If the pH is close to the pKa, an equimolar mixture of the two species will exist, resulting in a green color.
In this case, since the pH of the solution (3.8) is lower than the pKa (4.6), the acidic form (HIn) will predominate, and the color of the indicator will be yellow.
Therefore, your initial guess was correct - the indicator will be yellow in a solution with pH 3.8.