Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: RaelAerosolKid on January 09, 2021, 11:39:37 PM
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So my teacher gave us an exercise list and one of the questions (translated) was this:
"A solution (A) contains an alkaline mixture of Na2X 0.2000M [H2X (pKa1 = 5.5) and HX-
(pKa2 = 11.0)] and 0.2500 M NaY [HY (pKa1 = 11.0)].
A 10.00 mL aliquot of solution (A) was titrated against 0.2000 M HCl solution.
What volume of 1.5000 M HCl solution should be added to 2.00 L of solution (A) for
to obtain a buffer solution pH = 7.0?"
I was taught how to calculate the volume when I had a simple weak acid and salt, not this mess.
I thought about giving priority to the strongest acid, or to the first Ka, or the most concentrated salt but I never came to a solid conclusion. I have searched "buffer solution with mixture of 2 weak acid salts" and found nothing.
Is this question even solvable? xD
Thanks in advance
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I was taught how to calculate the volume when I had a simple weak acid and salt, not this mess.
Calculate it independently for both salts and add the volumes of acid.
What volume of 1.5000 M HCl solution should be added to 2.00 L of solution (A) for to obtain a buffer solution pH = 7.0?"
Frankly saying, chemists will not consider this solution after neutralization as a buffer solution. But roughly you can think of it as a buffer solution based on the acid dissociation constant Ka1.
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Don't know if this is against the rules, but could you be more direct in how to solve this question?
The first part makes total sense, thanks.
But when I think about the Henderson–Hasselbalch equation to find the pH,
(it would be "pH = Pka1 - log(([H2X] + moles of HCl)/([HX-] - moles of HCl))", right?)
I don't know the concentration of [H2X] or [HX-] to begin with so therefore I also wouldn't know what amount of HCl to add
Am I missing something? Damn this subject is hard
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Oh I think I have a clue, I just have to calculate the pH of the salt solution alone and then I think there is no need to do the Henderson–Hasselbalch equation.
So I would have, say, X2- from the salt + H2O -> HX- + OH-
That means I would have to use the Ka2 to then transform it into a Kb and then find the concentration
Is this correct?
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Solve for buffer H2X/NaHX with the sum of the concentration 0.2000 M, then neutralize all salts with HCl.
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Nope, didn't work. I found that a 2L solution of any of the 2 salts had a pH of about 12.19. That would require an absurdly low amount of HCl to be added to change the pH to 7
FU&*K.
(I wrote this before your reply)
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Solve for buffer H2X/NaHX with the sum of the concentration 0.2000 M, then neutralize all salts with HCl.
Ok so I did "7 = 5.5 - (log ([H2X]/[NaHX]))" and "[H2X] + [NaHX] = 0.2"
The result was that [H2X] = 0.00613 and [NaHX] = 0.19387
I thought about the reaction 2HCl + Na2X --> H2X + 2NaCl
Does that mean I would have to do stoichiometry and get a amount of 0.00613 of H2X if I want to get a pH of 7? (And then do the same in the other salt)
Sorry for the amount of questions, my teacher doesn't really explain this really well
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Neutralize NaY completely
Neutralize Na2X to NaHX
Then neutralize 0.19387 M to H2X should 0.006
Add all HCl used (remember about 2 liters of solution.
I estimated volume HCl between 0.85 - 0.87 L without a calculator. should be ~0.6
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Would this explanation assume that we have a reaction that is like "HCl + Na2X --> NaHX + NaCl" and not right away "2HCl + Na2X --> H2X + 2NaCl"?
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This is exactly how many reactions go.
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Would this explanation assume that we have a reaction that is like "HCl + Na2X --> NaHX + NaCl" and not right away "2HCl + Na2X --> H2X + 2NaCl"?
You had a very similar conceptual problem with Ca(OH)2 question. Protonation of X is a two step process which can be written as two consecutive steps or as an overall reaction. Both approaches have their applications, neither is "right" or "better" in general.
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Would this explanation assume that we have a reaction that is like "HCl + Na2X --> NaHX + NaCl" and not right away "2HCl + Na2X --> H2X + 2NaCl"?
You had a very similar conceptual problem with Ca(OH)2 question. Protonation of X is a two step process which can be written as two consecutive steps or as an overall reaction. Both approaches have their applications, neither is "right" or "better" in general.
Ok I think I managed to do it, thank you so much for your help
More helpful than my own teacher ;)