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Topic: calculate pH of 0,1M Na3PO4  (Read 7712 times)

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Offline xshadow

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calculate pH of 0,1M Na3PO4
« on: December 13, 2016, 11:40:29 AM »
Hi :)

I need a help for this:



I have to calculate pH of Na3PO4 using the CHARGE BALANCE...and using also some approximation
I lnow that there is this approximated formula : [OH-] = sqrt(CNa3PO4* Kb )

But I don't know how to arrive at something similar to this starting with the charge balance expression  (I assume that KbPO4 3- >> other Kb


And are the approximation done "?!" and "??" ,in that image, correct?
How can I continue from that expression ?

THanks very much :)

Offline AWK

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Re: calculate pH of 0,1M Na3PO4
« Reply #1 on: December 13, 2016, 12:09:08 PM »
Write down reaction for the first step of PO43- hydrolysis, then do approximation in Kb1 equation (neglect ion product of water Kw). This gives two approximation including yours.
AWK

Offline xshadow

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Re: calculate pH of 0,1M Na3PO4
« Reply #2 on: December 13, 2016, 02:07:34 PM »
Write down reaction for the first step of PO43- hydrolysis, then do approximation in Kb1 equation (neglect ion product of water Kw). This gives two approximation including yours.

Do you mean that I should do the charge balance only for this "system/reaction" ??:

PO43- + H2O <----->  HPO42- + OH-

Because I need to solve the exercise using the charge balance.... Can't I solve this pH calculation using that charge balance??

I should do some approxsimation in that expression...

Offline Borek

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Re: calculate pH of 0,1M Na3PO4
« Reply #3 on: December 13, 2016, 04:57:51 PM »
Charge balance alone is not enough, you need to use Ka (or Kb) values as well.

See if the method outlined here:

http://www.chembuddy.com/?left=pH-calculation&right=pH-polyprotic-acid-base

doesn't help. It can easily get simplified.
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