How to Determine Ka of Bromcresol Green from Absorbance Values?

[jpd5184]

Homework Statement

I need to find out the ka of bromcresol green for a 5 different solutions. I have the absorbance at the maximum wavelength for each solution. In the lab report the next box says

An-Ahb / Ab(negative ion) -An

i have the values for An,Ahb, and Ab but I am not sure if the equation is subtracting the values or multiplying by negative values

Homework Equations

The Attempt at a Solution

 

[Borek]

Please elaborate - what is what of what?

 

[jpd5184]

im not sure what your saying, what other information is needed.

 

[Borek]

What do you mean - Ka for 5 different solutions? Ka is constant for a given substance, so it should be identical for all solutions. I guess you mean "experimental value of Ka for each experiment".

What is An? I guess Ahb is meant to be A_HB, but you can't assume every symbol is obvious to everyone.

I guess your post means that you should now calculate value of the expression and you are not sure how to interpret this expression. I have the same problem with your post - it is not much more precise than the expression you posted

 

[jpd5184]

ill give you an overview of the lab.

we had to make 8 solutions. solution A then solutions 1-7. solution A had 25.00ml of HOAc dilluted to 100ml with water. this solution was but into a buret.

solution one has 5.00ml of bromcresol green(BCG) and 5.00ml NaOAc, diluted to 100ml.

we then took absorbance readings of solution 1 with a spectrophotometer to find out the maximum absorbance.

solution 2 then got 2.00ml of solution A added to it with the buret. Solution three got 2.00ml of solution A(for a total of 4.00ml) and so on till solution 7(which had 2.00ml of solution one added to it for a total of 12.00ml).

all solutions were made from the previous one and each solution was measured at the maximum absorbance except for 3 and 7 which were measured for the full scale of wavelengths to figure out there maximum absorbance readings.

 

[Borek]

Still no idea what is An.

I guess your calculations should be based on Henderson-Hasselbalch equation. You know pH (you forced it using acetate buffer), you have absorbances that can be used to calculate ratio of BCG both in the form of acid and conjugate base - that's enough to calculate pKa and Ka.

 

[jpd5184]

An = absorbance at maximum wavelength B-

[H30+] = Ka[HOAc/[OAc-]

 

[Borek]

I would concentrate on trying to understand whether and if - how

An-Ahb

and

Ab(negative ion) -An

are related to concentrations of BCG in the form of acid and conjugate base.

But I think I have already signaled it.