Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: LilyFFC on February 07, 2011, 12:58:11 PM
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We were given a lab in which we first standardized NaOH by titrating it with a solution containing KHC8H4O4 and phenolphthalein. The equivalence point was measured when the solution went from colourless to light pink.
We then took the standardized NaOH solution (0.113 mol dm-3) and titrated it with a solution of H2C2O4 - oxalic acid (0.0835 mol dm-3) and phenolphthalein. The equivalence point of this dissociation was found when the light pink solution went colourless.
From this, it is required that I find the Ka value of oxalic acid.
By using the dissociation H2C2O4 --> H+ + HC2O4- I have formed the following equation and have substituted [H2C2O4] = 0.0835 mol dm-3:
Ka = [H+] [HC2O4-] / [H2C2O4]
How would I go about finding the concentrations of the two unknown values? Or is there another way of doing so?
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well, first of all, what does the transition from pink to colorless mean?
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Would it not be an indicator of the equivalence point at which the amount of base in the solution is equal to the amount of acid?
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Indeed it indicates a transition point in the phenolphtalein, which pH does it indicate?
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I'm not sure I understand what you mean - we did not measure pH in this lab, at least not directly..
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an indicator is used exactly for that: it indicates the pH due to a color shift in the indicator when the solution they are in reaches a certain pH value.
So please again tell me the exact procedure you did when titrating the oxalic acid.
Which is the titrant? what was the analyte? How much volume?
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Okay.
50.0 mL of the oxalic acid was in a burette and was titrated into 25.0cm3 of NaOH in the conical flask below the burette. The NaOH had 2 drops of phenolphthalein in it. On average, 23.2 mL of the oxalic acid was required to reach the equivalence point.
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there you go.
Now, you know the concentration of the sodium hydroxide you said in your first post, right?
Or you determined it, either way I am going to assume you know it)
So you know the concentration of OH- in your solution, right?
then check this: http://en.wikipedia.org/wiki/Phenolphthalein (http://en.wikipedia.org/wiki/Phenolphthalein)
for you the transition that matters is the pink-to-colorless transition at higher pH, which occurs at a pH of about 8.2.
So now you know the original concentration of OH-, you know the amount of oxalic acid you added, and you know the end concentration of OH-
so you can calculate how many H+ of the oxalic acid was used to neutralize the NaOH.
which should give you the amount of C2O42- and HC2O4- you have left in solution. And from the pH you know the [H+]
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Okay, I think I'm understanding this more now...
Yes, I know [NaOH] - is that value equal to [OH-]initial?
And I'm not sure how to calculate the [OH-]end or the [H+]. Would it be imprecise to determine [H+] from the pH of the indicator at 8.2?
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I believe you have to estimate it from the indicator pH, else why add the indicator at all?
And yes, NaOH is a strong base that can be assumed to be totally dissociated.
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Sorry to jump in, but this sounds like the most idiotic lab I have seen in my life. If my calculations are OK determined pKa is over 4 units off. This approach can't give reasonable result, especially for oxalic acid.
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I agree with you, Borek. I have not calculated any values, but using a base-indicator for an acid pKa determination does not sound very sensible indeed.
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No, it doesn't make sense - it's just what my teacher has given us :/
So in order to determine the ka of oxalic acid, what calculation must I do? Why am I trying to find C2O42- and HC2O4-and how do I find [OH-]end?
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This might explain the use of phenolphthalein as an indicator:
http://ww.chemistry-react.org/go/textonly/Faq/Faq_30025.html
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So in order to determine the ka of oxalic acid, what calculation must I do? Why am I trying to find C2O42- and HC2O4-and how do I find [OH-]end?
Write expression for Ka2 (second stepwise dissociation constant), check what you know and what you don't know.
I agree with you, Borek. I have not calculated any values, but using a base-indicator for an acid pKa determination does not sound very sensible indeed.
That's not the main problem. If end point pH would be close to pKa2, it could be even an interesting experiment, but it is not, so error is out of control. I don't want to write too much, Lily is confused without detailed analysis ;) Perhaps I will add more later.
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Okay so the Ka2 = [H+][C2O42-] / [HC2O4-]
I know [H+] = 10-8.2
Would [HC2O4-] = [H2C2O4] ? And I'm not sure what [C2O42-] is equal to.
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from the values of your experiment you can calculate how much OH- reacted with protons from the acid.
for each molecule of oxalic acid between 1 and 2 protons have reacted. The first proton will have reacted from each of the oxalic acid molecules, the second from only a portion; you can calculate exactly from how many using the amount of OH- that reacted
so: how many mole of OH- has reacted?
how many moles of oxalic acid has reacted?
so from how many moles of oxalic acid did the second proton react as well?
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I still don't follow - how am I supposed to calculate how much OH- reacted and how much I started with?
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well, you know with how many moles of OH you started, right? (the number of moles of NaOH)
you know the pH at the end, and thus the concentration H+ at the end, right?
Do you remember the relation between [H+] and [OH-]? If you don't, then look it up and remember it!
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How much you had is simple - NaOH is fully dissociated. Just assume it it was a limiting reagent for teh neutralization reaction. Amount that is present in the solution at the equivalence point is so small it doesn't matter. Besides, you can't assume it is from NaOH, some of it comes from water autodisociation and oxalate anion hydrolysis.