finding Ka of various solutions

[jpd5184]

1. The problem statement, all variables and given/known data

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 im not sure if the equation is subtracting the values or multiplying by negative values

2. Relevant equations



3. The attempt at a solution

[Borek]

Please elaborate - what is what of what?

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[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 AHB, 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 :wink:

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[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 (http://www.chembuddy.com/?left=pH-calculation&right=pH-buffers-henderson-hasselbalch). 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.

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