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Titration, weak unknown acid with strong base! Help please..
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Topic: Titration, weak unknown acid with strong base! Help please.. (Read 19123 times)
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SaraW
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Titration, weak unknown acid with strong base! Help please..
«
on:
September 30, 2009, 07:56:44 AM »
I have a problem that needs solving...if anyone else has the time please take a look
Titration of a weak acid with a strong base. Im titrating an 50 ml of an unknown weak acid with an unknown concentration with NaOH (concentration NaOH 0,2 M) . I’ve been given a titration curve:
Im supposed to get the equivalence point out of this curve…Im guessing its between 8.5 – 9 (right?).
I need to calculate
the unknown weak acids initial concentration
and its
Ka(equilibrium constant)
.
Please help
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SaraW
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Re: Titration, weak unknown acid with strong base! Help please..
«
Reply #1 on:
September 30, 2009, 08:04:34 AM »
So.. Im thinking
C(acid)= (V NaOH)*(C NaOH)/(V(acid))
V NaOH: 75ml (from titration curve)
C NaOH: 0,2 M
V (acid): 50ml
0.2
* 0,075 = 0,3 M
0,05
Initial concentration of the weak acid is 0.3M
Does that sound right?
«
Last Edit: September 30, 2009, 08:15:52 AM by SaraW
»
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Borek
Mr. pH
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Re: Titration, weak unknown acid with strong base! Help please..
«
Reply #2 on:
September 30, 2009, 08:30:33 AM »
75 mL is wong - how many mL were added to reach end point pH you have mentioned?
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ChemBuddy
chemical calculators - stoichiometry, pH, concentration, buffer preparation,
titrations.info
SaraW
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Re: Titration, weak unknown acid with strong base! Help please..
«
Reply #3 on:
September 30, 2009, 08:36:30 AM »
did i mention an end point pH? is that the equivalence point in this case?
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SaraW
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Re: Titration, weak unknown acid with strong base! Help please..
«
Reply #4 on:
September 30, 2009, 09:02:21 AM »
Ok let me try again...
C(acid)= (V NaOH)*(C NaOH)/(V(acid))
V NaOH: 50ml
C NaOH: 0,2 M
V (acid): 50ml
0.2
* 0,05 = 0,2 M
0,05
Initial concentration of the weak acid is then 0.2M
50 mL NaOH when pH 8,5-9
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Borek
Mr. pH
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Re: Titration, weak unknown acid with strong base! Help please..
«
Reply #5 on:
September 30, 2009, 09:18:16 AM »
Much better now.
For general information:
http://www.titrations.info
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ChemBuddy
chemical calculators - stoichiometry, pH, concentration, buffer preparation,
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SaraW
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Re: Titration, weak unknown acid with strong base! Help please..
«
Reply #6 on:
September 30, 2009, 09:49:57 AM »
when it comes to the equilibrium constant
pKa(acid) = pH = 2,9
10^-pKa(Acid) = 10^-2,9 = 1.26*10^3
right?wrong?
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Borek
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Re: Titration, weak unknown acid with strong base! Help please..
«
Reply #7 on:
September 30, 2009, 10:18:10 AM »
No idea what you are doing, why, nor what for.
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ChemBuddy
chemical calculators - stoichiometry, pH, concentration, buffer preparation,
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SaraW
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Re: Titration, weak unknown acid with strong base! Help please..
«
Reply #8 on:
September 30, 2009, 10:21:35 AM »
trying to get the Ka(equilibrium constant)
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SaraW
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Re: Titration, weak unknown acid with strong base! Help please..
«
Reply #9 on:
September 30, 2009, 10:54:57 AM »
ok so its wrong....any suggestions on how to make it better then?
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Borek
Mr. pH
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Re: Titration, weak unknown acid with strong base! Help please..
«
Reply #10 on:
September 30, 2009, 12:06:02 PM »
There is a characteristic place on the titration curve where pH=pKa.
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ChemBuddy
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SaraW
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Re: Titration, weak unknown acid with strong base! Help please..
«
Reply #11 on:
September 30, 2009, 12:11:06 PM »
Ok i see 8.5-9.
i thought it was when NaOH startedbeing added(2.9).
does this then make sense..or is the way im calculating it wrong also..
pKa(acid) = pH = 8.5
10^-pKa(Acid) = 10^-8.5 = 3.16*10^9
Ka= 3.16*10^9
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Borek
Mr. pH
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Re: Titration, weak unknown acid with strong base! Help please..
«
Reply #12 on:
September 30, 2009, 01:56:59 PM »
No, its is not pH at the end point (nor equivalence point).
Do you know
Henderson-Hasselbalch equation
?
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ChemBuddy
chemical calculators - stoichiometry, pH, concentration, buffer preparation,
titrations.info
SaraW
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Posts: 22
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Re: Titration, weak unknown acid with strong base! Help please..
«
Reply #13 on:
September 30, 2009, 02:32:22 PM »
pH = pKa + log10
[conjugate base]
[conjugate acid]
how can i get pKa if I dont know which acid it is?
i know im dense...but i need a bit more help
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SaraW
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Re: Titration, weak unknown acid with strong base! Help please..
«
Reply #14 on:
September 30, 2009, 03:17:27 PM »
pKa = −log10Ka
So pKa is halfway to the equivalence point?
4.25= pKa?
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Titration, weak unknown acid with strong base! Help please..