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Topic: pH of a solution  (Read 483 times)

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Offline smbgfull

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pH of a solution
« on: April 15, 2021, 11:00:59 AM »
How a solution made of 20 mL of NaOH, 10 mL of HCl, 10 mL NaH2PO4 and 5 mL of Na3PO4, all of them in a concentration of 10 mmol/L, would behave? I already tried every way i can think of, but none give the solution of the pH of this system.

Offline chenbeier

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Re: pH of a solution
« Reply #1 on: April 15, 2021, 12:43:09 PM »
It is a buffer. You have to calculate how much and which phosphate species you have then you have to use the Henderson Hasselbalch equation .You need the right pKa value.

Offline Borek

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Re: pH of a solution
« Reply #2 on: April 15, 2021, 06:19:43 PM »
This is tricky, PO43- is too strong base for HH equation.
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Offline chenbeier

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Re: pH of a solution
« Reply #3 on: April 16, 2021, 01:09:16 AM »
Instead of HH, what is your solution. After mixing 100 Mikromole Na2HPO4 and 50 mikromole Na3PO4 in 45 ml total volume is present.

Is at dissociation  to PO4 3- + H2O => HPO4 2- + OH-

Offline Babcock_Hall

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Re: pH of a solution
« Reply #4 on: April 16, 2021, 10:00:07 AM »
I wonder whether or not there might be a way to obtain an approximate answer:  Start by conceptually letting the strongest acid and strongest base react to completion, and do this in an iterative manner.  You will probably have to make a correction for ionic strength.

Can you show what you have tried, so far?

Offline Orcio_Dojek

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Re: pH of a solution
« Reply #5 on: April 17, 2021, 11:48:27 AM »
@smbgfull
Quote
I already tried every way i can think of, but none give the solution of the pH of this system.

I think you should aproximate that only Na3PO4 undergo hydrolisis, as taking hydrolisis of Na2HPO4 into account will provide cubic equation !!!

I know it is big aproximation, but I think this is the only way to solve this problem.

Rest is similiar as here: https://www.chemicalforums.com/index.php?topic=107568

With exception that hydrolysis will reduce concentration of Na3PO4.

(my result is pH = 10,55)
« Last Edit: April 17, 2021, 12:24:49 PM by Orcio_Dojek »

Offline Borek

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Re: pH of a solution
« Reply #6 on: April 17, 2021, 01:36:24 PM »
(my result is pH = 10,55)

And it is off by over 0.4 unit.
« Last Edit: April 17, 2021, 04:36:09 PM by Borek »
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Offline Orcio_Dojek

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Re: pH of a solution
« Reply #7 on: April 17, 2021, 05:40:36 PM »
Is it because of typo in calculations or because of hydrolysis of Na2HPO4?

After validation my answer is pH = 10,31, and - as of hydrolysis of Na2HPO4 - MAYBE it can be calculated, but this is very time-consuming, that's why I take it as aproximation.

Offline Borek

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Re: pH of a solution
« Reply #8 on: April 18, 2021, 03:30:57 AM »
It is because the system is way too complicated for most simplifications to work.

You can always check your result just by plugging numbers you got into all equilibria and checking if the concentrations found produce expected Ka values.

Your new answer is even worse than the previous one.
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Offline Orcio_Dojek

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Re: pH of a solution
« Reply #9 on: April 18, 2021, 05:38:57 AM »
You are right, it was just another typo, pH = 10,99 is the correct value (hydrolysis constant = 0,025, hydrolysis of Na2HPO4 omitted).
« Last Edit: April 18, 2021, 07:20:41 AM by Orcio_Dojek »

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