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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: xshadow on December 13, 2016, 11:40:29 AM

Title: calculate pH of 0,1M Na3PO4
Post by: xshadow on December 13, 2016, 11:40:29 AM

Hi :)

I need a help for this:
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fi66.tinypic.com%2F291oy28.jpg&hash=14e0f730850898439715164381e1da6ecc0e5fef)


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(C_Na3PO4* K_b )

But I don't know how to arrive at something similar to this starting with the charge balance expression  (I assume that Kb_{PO4 3-} >> other Kb


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

THanks very much :)

Title: Re: calculate pH of 0,1M Na3PO4
Post by: AWK on December 13, 2016, 12:09:08 PM

Write down reaction for the first step of PO₄^3- hydrolysis, then do approximation in K_b1 equation (neglect ion product of water K_w). This gives two approximation including yours.

Title: Re: calculate pH of 0,1M Na3PO4
Post by: xshadow on December 13, 2016, 02:07:34 PM

Quote from: AWK on December 13, 2016, 12:09:08 PM

Write down reaction for the first step of PO₄^3- hydrolysis, then do approximation in K_b1 equation (neglect ion product of water K_w). This gives two approximation including yours.

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

PO₄^3- + H₂O <----->  HPO₄^2- + 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...

Title: Re: calculate pH of 0,1M Na3PO4
Post by: Borek 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.