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### Topic: How to calculate concentrations using titration curve?  (Read 23731 times)

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#### lollipopsbing

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##### How to calculate concentrations using titration curve?
« on: March 26, 2012, 12:27:36 PM »
This week in labs we titrated maleic and fumaric acid using NaOH and created a titration curve. Now we are being asked to calculate the concentrations of both acids in the solution and I'm a little confused.

Would I use the inflection point of the titration graph to get the pH and then find the Ka value of the solution? and then from there would I solve for the concentration using the equilibrium equation? The only problem I have with that is I don't know what the molarity of the maleic/fumaric acid is because they didn't give it to us. Does anyone know another way I could get the concentration?

#### Borek

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##### Re: How to calculate concentrations using titration curve?
« Reply #1 on: March 26, 2012, 01:15:55 PM »
Do you know concentration of NaOH used?

Do you know what is pH of a mixture containing equimolar concentrations of an acid and its conjugate base?
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#### cakaro13

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##### Re: How to calculate concentrations using titration curve?
« Reply #2 on: March 26, 2012, 01:23:56 PM »
If I understand what you are asking correctly, you should relate the amount/concentration of NaOH added to the pH, pKa, and the conc. of acid leftover. Look up the Henderson-Hasselbalch equation, and see how you can relate it to the equivalence point and the half-equivalence point on the titration curve.

#### lollipopsbing

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##### Re: How to calculate concentrations using titration curve?
« Reply #3 on: March 26, 2012, 02:10:30 PM »
borek: yes, I know that we used .100 M NaOH, but i don't know what that pH is, no... using it though would give you Ka, right?

cakaro: oh ok, but which pKa value would I use? 1 or 2?

thanks both of you!

#### Borek

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##### Re: How to calculate concentrations using titration curve?
« Reply #4 on: March 26, 2012, 02:16:01 PM »
So you have no idea what was the purpose of the experiment?

http://www.titrations.info/titration-calculation
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#### DrCMS

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##### Re: How to calculate concentrations using titration curve?
« Reply #5 on: March 26, 2012, 02:22:22 PM »
Isn't the key here that both acids are dicarboxylic acids and therefore if you know the concentration of the NaOH you can using the shape of the titration curve calculate the concentrations of each.

#### lollipopsbing

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##### Re: How to calculate concentrations using titration curve?
« Reply #6 on: March 26, 2012, 02:31:01 PM »
Borek I think I just misunderstood your question. I know that at the equivalence point, the pKa value is equal to the PH of the solution but I don't understand how that would relate back to concentration other than the way I thought of originally.

But if I use the molarity of NaOH instead and use the henderson-hasselbalch equation then I would just solve for the base concentration that way? Is that what cakaro13 was getting at?

DrCMS: Is there a way to do that solely by looking at the curve?

#### DrCMS

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##### Re: How to calculate concentrations using titration curve?
« Reply #7 on: March 26, 2012, 03:19:21 PM »
DrCMS: Is there a way to do that solely by looking at the curve?

What does a titration curve for a diacid have two of?

#### Borek

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##### Re: How to calculate concentrations using titration curve?
« Reply #8 on: March 26, 2012, 04:35:25 PM »
I know that at the equivalence point, the pKa value is equal to the PH of the solution

pH is not equal to pKa at equivalence point, this is completely off. pH equals pKa at other characteristic point.

Quote
But if I use the molarity of NaOH instead and use the henderson-hasselbalch equation then I would just solve for the base concentration that way?

No, this is again off.

Have you read the page I linked to? You may also want to take a look at the main page of the site: http://www.titrations.info/
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info