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Topic: Titration Calculation w/ Sodium Acetate  (Read 29623 times)

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sundrops

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Re:Titration Calculation w/ Sodium Acetate
« Reply #15 on: October 27, 2005, 03:03:22 AM »
hey AWK,

I need to determine the pH of the solution at the equivalence point, and then compare my answet  to the pH of a 0.0667M CH3COOH solution.

i can determine the pH of the 0.00667M CH3COOH (pH=2.97) but I'm still lost about the pH of my CH3COONa solution.

do you have any ideas on how to go about doing that?

sundrops

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Re:Titration Calculation w/ Sodium Acetate
« Reply #16 on: October 27, 2005, 03:22:39 AM »
could I write the equation like:

  CH3COOH + H2O <-> CH3COO- + H3O
I     0               0              0.2           0.1
C    +x                              -x            -x
E     x                            0.2-x         0.1-x

Ka = (o.2-x)(0.1-x) / x
 using the quadratic formula  x=0.1

so [H+] = 0.1-x
so [H+] = 0
so that means that there is no pH. because -log(0) doesn't exist.
so does that mean that the reaction goes to completion? but in that case wouldn't the pH be 7?

I'm a little lost.

help?

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Re:Titration Calculation w/ Sodium Acetate
« Reply #17 on: October 27, 2005, 03:52:29 AM »
What is ICE table?

Initial, Change, Equlibrium - a way of dealing with equilibrium problems. Sundrops already used it later in the thread.

Quote
Wouldn't the pH be 7 if it is a neutralisation?

No, but you have already found that.
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Offline Borek

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Re:Titration Calculation w/ Sodium Acetate
« Reply #18 on: October 27, 2005, 04:06:45 AM »
The more I read through the thread the less I understand.

For titration of sodium acetate with hydrochloric acid you cant use phenolopthtaleine, as the final pH will be acidic.

There are three areas of the curve. I will outline very shortly how how to deal with them, check much more detailed explanation on titration curve calculation on my site.

pKa value of acetic acid is known and given (as AWK wrote) in physicochemical tables. Consider you know the value, it is about 4.75. The calculations can't be done without knowing Ka.

At first you have solution of sodium acetate only. Acetic anion will react with water producing OH- ions, thus making solution slightly basic. To calculate pH you have to:
1. Calculate pKb of the acetic anion (pKa+pKb=pKw, acetic anion is a conjugated base of acetic acid)
2. Use ICE table to calculate OH- (using Kb and ICE table)
3. Convert pOH to pH.

On the curve you have a mixture of acetic acid and acetic anion - so it is a buffer solution. Assume that all added hydrochloric acid reacted with acetic anion  and from simple stoichiometry calculate concentrations of both forms, then use Henderson-Hasselbalch equation

At the endpoint all of the acetate was converted to acetic acid, so you have to calculate pH using known concentration of weak acid (don't forget the dilution factor from titration), its Ka value and ICE table.
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sundrops

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Re:Titration Calculation w/ Sodium Acetate
« Reply #19 on: October 27, 2005, 04:33:58 AM »
thx Borek, that was a HUGE help.

I think it was the initial equation that completely threw me off because I couldn;t formulate an ice table with the HCl and NaCl's floating around.

Ok well here's what I did, (followed your instructions to the letter!  :D)

pKa = 4.75
Ka=1.78E-5

Kb = 10E-14 / 1.78E-5
Kb = 5.62E-10

then I made my ICE table;
    CH3COO- <-> OH- + CH3COOH
I       0.2              0           0
C         -x             +x        +x
E     0.2-x             x            x

Kb = [OH-][CH3COOH] / [CH3COO-]

x2 + 5.62E-10x - 1.124E-10 = 0
use quadratic formula and.... x=1.06E-5
therefore [OH-] = 1.06E-5
pOH = -log[OH-]
pOH = 4.97

14-pOH = pH
pH = 9.025

does that look ok?
I feel pretty confident about it, mainly because it makes more sense now  ;)


Offline Borek

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Re:Titration Calculation w/ Sodium Acetate
« Reply #20 on: October 27, 2005, 04:51:52 AM »
OK
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sundrops

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Re:Titration Calculation w/ Sodium Acetate
« Reply #21 on: October 27, 2005, 04:54:26 AM »
wait a second,

wouldn't  CH3COO- <-> OH- + CH3COOH work for CH3COOH being titrated with NaOH rather than CH3COONa being titrated with HCL?

so shouldn't the equation be: CH3COOH + H2O <-> CH3COO- + H3O+ ??

sundrops

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Re:Titration Calculation w/ Sodium Acetate
« Reply #22 on: October 27, 2005, 05:27:57 AM »
nvmnd - i totally get it now.

thx everyone!  ;D

Offline Borek

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Re:Titration Calculation w/ Sodium Acetate
« Reply #23 on: October 27, 2005, 05:33:59 AM »
Titration of acetic acid with NaOH:

CH3COOH + OH- <-> CH3COO- + H2O

Such titration makes sense, as the pH change near end point is large and easy to detect.

Titration of acetate with HCl:

CH3COO- + H+ <-> CH3COOH

Such titration will be not usefull, as the titration curve is very flat (see the picture).

Botth equations describe some equilibria, but they also give you general idea about the direction of the process taking place during titration.
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sundrops

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Re:Titration Calculation w/ Sodium Acetate
« Reply #24 on: October 27, 2005, 07:54:22 AM »
how do you make graphs like that Borek? It's VERY helpful, is it a downloadable program? its great.

Offline Borek

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Re:Titration Calculation w/ Sodium Acetate
« Reply #25 on: October 27, 2005, 08:08:59 AM »
how do you make graphs like that Borek? It's VERY helpful, is it a downloadable program? its great.

BATE - link in my signature.
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Offline mike

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Re:Titration Calculation w/ Sodium Acetate
« Reply #26 on: October 27, 2005, 07:17:51 PM »
Quote
pH = 9.025

Borek, does this mean that the end point of the titration of acetate with HCl is basic?

I don't understand ???
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Offline Borek

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Re:Titration Calculation w/ Sodium Acetate
« Reply #27 on: October 27, 2005, 07:32:28 PM »
Borek, does this mean that the end point of the titration of acetate with HCl is basic?

No, it is pH of the sodium acetate solution, BEFORE titration.
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Offline mike

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Re:Titration Calculation w/ Sodium Acetate
« Reply #28 on: October 27, 2005, 08:00:22 PM »
Quote
No, it is pH of the sodium acetate solution, BEFORE titration.

Ok this makes more sense :)


Quote
Write out the equation, calculate the volume HCl required and calculate the pH of the solution at its equivalence point.

So is there a way to work out the pH at the equivalance point? With the data provided with the question?

Would this mean then that the phenolthalein would have to change from red to colorless for this endpoint?

Sorry for the endless questions, please don't die like the magic goldfish after my three questions ;) ;)
There is no science without fancy, and no art without facts.

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Re:Titration Calculation w/ Sodium Acetate
« Reply #29 on: October 27, 2005, 08:06:50 PM »
So is there a way to work out the pH at the equivalance point? With the data provided with the question?

Without Ka - no. With Ka - yes. At the equivalence all acetate have reacted with HCl so it is in form of acetic acid. All you have to do is to calculate pH of acetic acid solution (taking dilution into account).

Quote
Would this mean then that the phenolthalein would have to change from red to colorless for this endpoint?

Look at the titration curve I have posted earlier. Phenolophtaleine color will be visible at the very beginning of the titration, when only acetate is present. First drop of HCl will make the solution colorless.

Quote
Sorry for the endless questions, please don't die like the magic goldfish after my three questions

I am not going to die, but soon I am going to sleep. It is 2 a.m. here.
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