[Adam_ko]

Hello, ive got problem with calculating of Absorbance from the pH.

Homework - weak acid ( Ka=4,8e-6) with concentration 8*10e-5 mol/l in cuvette with thickness 1cm in solution with pH= 1 had absorbance 0,658 and in solution with pH=13 had absorbance 0,076. Calculate absorbance at pH=4,7. Wavelength is 450 nm.

I have been trying to solve this problem for 2 hours atleast, but i am unable to solve it  

[chenbeier]

What did you tried so far.
Tip: Formula pH for weak acids, additional Law of Lambert Beer.

[Borek]

Think what are concentrations of acid and conjugate base in the solution at each listed pH.

[Adam_ko]

So, ive tried many things, for example Handerson-Hasselbach equation. However, when i put in the pH 13, the concetration of conjugate base is negative :/

Im doing something wrong but i cannot figure out what is it...

[Borek]

Show what you are doing and we will start from there.

You can estimate concentrations at the extremes even without calculations.

[Babcock_Hall]

A graph of A vs. pH might be a good place to start, just to get a feel for the problem.

[Adam_ko]

I dont know if i solved it correctly, but i tried to calculate it from the graph equation from excel - on the X axis i put pH and on Y axis i put Absorbance, so the graph equation was:
y=-0.0647x+0.9167
So i plugged in pH 4.47 for my pH and y/absorbance should be 0.627
what do you think?

[Borek]

Dependence is not linear, so your answer is most likely wrong.

You were given several hints suggesting how to find concentrations of the acid/base in the solution, have you tried them?

What is pKa of the acid?

[Adam_ko]

pka = -log Ka=-log(4,8e-6)=5,32

And yes, ive calculated molar absorption coefficient (molar attenuation coefficient) (with Lambert-Beer Law) for both pH, but i dont know what to do with those 2 numbers...

[Borek]

So the pH given is rather close to the pKa, isn't it?

And yes, ive calculated molar absorption coefficient (molar attenuation coefficient)

Molar absorption coefficient of what? (that's actually an important hint)

[Adam_ko]

Molar absorption coefficient of dissociated form of acid at pH 1?

[Babcock_Hall]

@OP, Does only one protonation-state absorb light, or do both the conjugate acid and the conjugate base absorb light?

[Borek]

Adam, you are playing hide and seek with us, it won't work. You ignore half of the questions, you answer only the part that fits you, and you are not moving ahead at all, we are all just wasting time.

You have a solution of a weak acid. What are concentrations of HA and A^- forms at pH 1?

If you will not attempt to answer that question precisely, I am going to lock the thread as it is going nowhere.

[Adam_ko]

Okey, ill try to answer every thing, but im desparate

1) So yes, the pKa is close to the pH (4,7) of the solution of the unknown absorbance.

2) To be honest, im not certailny sure, but i would guess that both conjugate base and conjugate acid both absorb light.

3) So the concentration of [A-] is the same as the concentration of [H+] which I calculated with quadratic formula to have a precise value... so the concentration of the [HA] can be calculated c=[HA]+[A-] -> [HA]=c-[A-]

[Babcock_Hall]

@OP, I believe that you are correct about 2.  However, I will drop out of the conversation for the moment, because it may be more difficult for you to answer questions from two people than questions from one.