PH of a polyprotic acid using multiple equilibria - is this correct?

[FredericChopin]

Homework Statement

Find the pH of 0.02025 M tartaric acid using the method of multiple equilibria given that its K_a1 value is 9.2*10^-4 and its K_a2 value is 4.31*10^-5.

Homework Equations

K_aoverall = K_a1 * K_a2

The Attempt at a Solution

Please check the Word document.

Is my method and calculation correct?

Thank you.

 

[Borek]

For some reason I can't open the file. .docx is not the best format. Try to post it here using LaTeX, or - at worst - as an image.

 

[FredericChopin]

Hm... that's strange.

I'll upload images files then (LaTeX is too much trouble...)

They're in order of page number.

 

[Borek]

Seems like you have completely ignored presence of HT^-.

pH of the 0.02025 M tartaric acid is 2.41, as long as your calculations don't reproduce this result they are wrong.

 

[FredericChopin]

The reason I'm asking is because I answered the same question but instead used the step-by-step equilibrium method and I got a different answer.

I can't see where I went wrong though. Both methods should yield the same answer.

They're in order of page number.

 

[FredericChopin]

Seems like you have completely ignored presence of HT^-.

pH of the 0.02025 M tartaric acid is 2.41, as long as your calculations don't reproduce this result they are wrong.

Hm... but wouldn't it cancel out in the equation?

 

[Borek]

No. Just because it cancels out in a partial system of equations doesn't mean it shouldn't be taken into account. Overall equation doesn't say a word about intermediate steps, which doesn't mean they don't exist.

Correct derivation is shown on the page I already linked to.

 

[FredericChopin]

Ah. You're right. I see where I went wrong.

It's not working because I'm writing an overall reaction that doesn't exist. And it wouldn't work unless there's a third reaction. Otherwise, as you said, I would miss the intermediate reactions.

Thank you.