ph Calculations....of NH4H2PO4 and KH2PO4

[angelguy]

Hi , This is Ashmit

I have been solving this problem since almost a week now, however, this stuff is getting more and more complicated.

The object is to determine the ph of KH2PO4 and NH4H2PO4 (mono basic) . I have done lab on both of these by taking known amount of salts and got 4.5 for KH2PO4 and 4.4 for other one.In this lab, I have taken 0.1156M of these compounds and dissolved it in 1 liter of water(pH 7) and then measured the pH. @ 25°C

These days my major concentration has been in the calculation of KH2PO4 since i think it will be simpler to deal with. I have assumed the complete solubility of KH2PO4 . and the key ion affecting the pH is dihygrogen phosphate H2PO4(-) . This gets dissociated into HPO4(--) and proton. with pKa value of 7.2. Although it further dissociate but not a big deal since pKa value of 12.35. I might have tried maximum possible methods I know but I cant get the close agreement with the lab value.
I have also used Henderson equation and it yield me the answer of 4.2

This is what i did,
pH = Pka + log([HPO(--)]/[H2PO4(-)] and Pka is 7.2 also I hv assumed that the conc of H2PO4(-) is 0.1156 M by stoichiometry.
and thereby conputed the value of HPO(--) by this formula . .
ka = x^2/(0.1156 - x) and got x = 9.89 * 10^(-5) aproxx which is the value of HPO(--).

Kindly guide me in this situation and also give me suggestion on how should i approach NH4H2PO4. situation. I will really appreciate if any one of you guys help me out since i am gonna get crazy doing one thingy over and over again. :?: Also had a confusion wrt considering NH4(+) in other situation. Please tell me mistakes in calculation.. which i m sure i did.

thanks a bunch..

Ashmit