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Offline johnsmith123

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Quantitative Analysis Acid Base
« on: November 04, 2016, 07:22:40 PM »
Need help on this question.
50.00 mL of an aqueous solution of which is 0.0500 M in ethylenediamine is mixed with 10.00
mL of an aqueous solution which is 0.400 M of perchloric acid. Calculate the pH at equilibrium.
Ethylenediamine Kb,1 = 4.33 x 10 -4
Kb,2 = 7.04 x 10 -8
Chlorous ka: 1.12 x 10-2

Offline magician4

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Re: Quantitative Analysis Acid Base
« Reply #1 on: November 04, 2016, 08:48:13 PM »
what kind of substance is ethylenediamine ? and what about perchloric acid ?
what will happen if you mix those two?
taking into account the amounts/conc. mentioned, what kind of system will result here? what formula belongs to those systems?

hint: the acid mentioned in the text ( HClO4) doesn't match the ka given below , as the ka given belongs to another acid (HClO2)


regards

Ingo
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Offline johnsmith123

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Re: Quantitative Analysis Acid Base
« Reply #2 on: November 06, 2016, 10:27:32 PM »
I understand that this is an acid base reaction with hclo4 being a strong acid and ethylenediamine being a weak base. by mixing the two a strong base and  weak acid will form with unknown conc. of x at equilibrium. i just do know where to start when given the initial conc. of both the initial acid and base

Offline Borek

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Re: Quantitative Analysis Acid Base
« Reply #3 on: November 07, 2016, 02:50:50 AM »
Hint: find the limiting reagent.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline magician4

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Re: Quantitative Analysis Acid Base
« Reply #4 on: November 07, 2016, 07:18:24 AM »
Quote
(...) by mixing the two a strong base and  weak acid will form with unknown conc. of x at equilibrium. (...)
no: the reaction of the (super-)acid HClO4 will lead to an ultraweak base ( i.e. ClO4- , considered to be so weak that you can treat it as neutral, for all practical purposes)

the part with "weak acid" (resulting from ethylene diamine) is correct, however.


hint: what does ethylenediamine mean, i.e. how much base is there around initially?
compare this to the initial amount of acid around!
what kind of system will result?


regards

Ingo


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Offline AWK

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Re: Quantitative Analysis Acid Base
« Reply #5 on: November 07, 2016, 08:46:09 AM »
I understand that this is an acid base reaction with hclo4 being a strong acid and ethylenediamine being a weak base. by mixing the two a strong base and  weak acid will form with unknown conc. of x at equilibrium. i just do know where to start when given the initial conc. of both the initial acid and base
We still do not know which acid you use: HClO4 or HClO2.
Please, explain.
Follow Borek advise and calculate stoichiometry of neutralization (with limiting reagent). In stoichiometric calculations (in both cases) assume no equilibrium.
AWK

Offline johnsmith123

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Re: Quantitative Analysis Acid Base
« Reply #6 on: November 07, 2016, 09:14:22 PM »
Thank you for the responses. Hclo4- is being used. I used the tips and i found oh- at equilibrium to equal .00104 mol using an ice table and the H+ conc. to be .004 mol. So .004 - .00104 = .00296 mol. I then found the conc. by .00296mol/.06 L and i took the -log of that and got 1.31 as the pH. Is that the correct answer?

Offline magician4

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Re: Quantitative Analysis Acid Base
« Reply #7 on: November 07, 2016, 09:58:50 PM »
Quote
(...) Is that the correct answer?
no

... and frankly speaking, neither do I understand what you've been calculating, nor do I understand you approach in general

you have a strong acid (and a certain amount thereof), you have a two-stage weak base (with, again, a certain amount thereof)
start by figuring out what will happen, and to what degree

after that, pls. come back with a detailled calculation of the resulting situation, so we can move further from there

regards

Ingo
There is a theory which states that if ever anybody discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another theory which states that this has already happened.
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Offline johnsmith123

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Re: Quantitative Analysis Acid Base
« Reply #8 on: November 07, 2016, 11:03:14 PM »
alright i think i got it. I just want to know if the answer i got is correct. Is it 12.4?

Offline johnsmith123

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Re: Quantitative Analysis Acid Base
« Reply #9 on: November 07, 2016, 11:43:47 PM »
disregard the last one, i actually got the answer to be 7.32, is this correct?

Offline magician4

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Re: Quantitative Analysis Acid Base
« Reply #10 on: November 08, 2016, 02:20:07 AM »
no

regards

Ingo
There is a theory which states that if ever anybody discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another theory which states that this has already happened.
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Offline AWK

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Re: Quantitative Analysis Acid Base
« Reply #11 on: November 08, 2016, 02:27:02 AM »
Show your stoichiometry in details.
AWK

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