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