[xshadow]

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(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

[AWK]

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.

[xshadow]

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...

[Borek]

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.