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

1. Jul 9, 2014

### FredericChopin

1. The problem statement, all variables and given/known data
Find the pH of 0.02025 M tartaric acid using the method of multiple equilibria given that its Ka1 value is 9.2*10-4 and its Ka2 value is 4.31*10-5.

2. Relevant equations
Kaoverall = Ka1 * Ka2

3. The attempt at a solution
Please check the Word document.

Is my method and calculation correct?

Thank you.

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• ###### Is this correct.docx
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2. Jul 10, 2014

### Staff: Mentor

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.

3. Jul 10, 2014

### FredericChopin

Hm... that's strange.

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

They're in order of page number.

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• ###### Is this correct-3.jpg
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4. Jul 10, 2014

### Staff: Mentor

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.

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5. Jul 10, 2014

### 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.

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• ###### pH Calculations 2-4.jpg
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6. Jul 10, 2014

### FredericChopin

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

7. Jul 10, 2014

### Staff: Mentor

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.

8. Jul 10, 2014

### 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.