[busque00]

Hey guys

I have a problem and I need your *delete me*
I'm quite confused and I don't know even what they're askin me about

1. An unkown organic acid was isolated from urine and then analyzed. 150mg sample was titrated with 0.05N NaOH (f=1.1). 3.6ml NaOH elevated the pH to 4.3, 8.0ml NaOH to 5.35. Calculate the dissocation constant if the equivalent weight of the acid is 300.

[Borek]

I can't see degree of dissociation in the question.

Do you know what dissociation constant is?

Can you calculate number of moles of acid from the given data?

[busque00]

How can I calculate K of that acid if I don't have [H+]?

[Borek]

Concentration of H⁺ is given. What is pH definition?

But IMHO there is something wrong the question. Data given allows calculation of two different values of the pKa, values that differ by 0.2.

Do you have an idea (from your course) what f=1.1 is supposed to mean? I have never seen such a symbol used to give an information in such a context, and even ignoring fact that I have never seen a symbol, I have no idea what it could be - 0.05N NaOH is just a 0.05N NaOH, no need for additional parameters.

[busque00]

f means factor:

so what I should do now is....

[Borek]

This is crazy, I have never seen such an approach, and I have read or at least browsed dozens of analytical chemistry books.

Assume acid was neutralized quantitatively by added base (just correct amount of base added using f), that will let you calculate concentration of acid and its conjugate base. Combine it with known concentration of H⁺ and you know everything required to calculate Ka.

[busque00]

so calculate concentration is equal 0.495M? yes? what about [H+]? I don't understand it at all.
the answer should be K=3.3 x 10-5

[Borek]

No, concentration is not 0.495M, no idea how you got it.

To calculate [H⁺] use pH definition.

Can you write formula for Ka? Can you identify acid and conjugate base in the formula?

[busque00]

so which pH should I use to calculate [H+]? 4.3 or 5.35?

ps. czy 300 to jest po prostu masa molowa? tj. 300 g/mol? bo nie mam jednostek i trochę jestem zdezorientowana?

[Borek]

so which pH should I use to calculate [H+]? 4.3 or 5.35?

Both - you will get two values. In theory they should be identical, can be they will differ because of experimental errors.

ps. czy 300 to jest po prostu masa molowa? tj. 300 g/mol?

Tak to należy rozumieć.

[busque00]

should I calculate C from the formula above (using this f) or just from C=n/V (C=m/M*V) ?

[Borek]

f is for NaOH solution, not for acid.

I wonder what you are going to use for V.