[Borek]

Originally Posted by angelguy

I have already downloaded your BATE and tried working the same compounds( KH2PO4) in it, however, I got pH = 4.07 for 0.11565M concentration. This value is pretty off from the experimental results pf 4.5 . I was kinda wondering , like did u guys considered the activity coefficient of all the ions in the software ?? Because after considering the activity coefficient of the pH my answer comes like 4.35 which is much closer to experimental result 4.5

You must have done something wrong. Start BATE and select phosphoric acid and potassium hydroxide as present in the solution. Enter 0.1156 for both concentrations - this is equivalent of KH2PO4. pH is calculated by the program as 4.69. There is a drop down list at the bottom of the dialog window - by default it says "ignore I". Select "calculate I" - and the pH calculated changes to 4.44 (note ionic strength of the solution right to the drop down list).

I really cant figure out why the pH(exp) of KH2PO4 and NH4H2PO4 are different for same concentration of correspoding salts. ?? because i was really confused, i performed the lab again doing the same thing but i got the same results. Mathemethically, I got the same answer for the NH4H2PO4 pH as KH2PO4, if I were to neglect the effect of NH4+. which is quite obvious. ..

It is enough if the salts used for preparation of solutions are not 100% pure - the pH measurements can be repeatable, but slightly different from expected. 0.1 pH unit is not a large difference (well, sometimes it is, I don't know what pehameter you used - note that if you have measured pH with one decimal digit accuracy it is enough if both solutions pH differ by 0.01 to display different values).

the reference you provided was extremely helpful thanks a lot for that. It helps me a lot in carrying calculations. But I cant really find the actually calculation of KH2PO4.

Look for pH = (pKa1 + pKa2)/2, perhaps it was presented for diprotic acid, not for triprotic - but it is the same approach. Third proton in phosphoric acid is nailed :wink:

Finally, Also my ultimate aim is to mix all different salts and then have to determine the ionic species at different pHs in a container mathemetically. I can name some of the important salts in the container.. 1) NH4H2PO4, MgSO4, NaCl, CaCl2 and so on.. I will be thankfull to you if you can also guide me about the approach to be taken to complete this query. Hoping for your favorable response again..

That's not so easy - there is no universal method that can be always safely used. In general, you should write down all equations describing the solution (mass balances, charge balance, water ioniozation and all dissociatian equilibria) ang go from there - some sums can be approximated by neglecting smaller values. Some substances can be treated as dissociated 100%. But it is always intuition and every assumption have to be later checked whether it holds.

Best,
Borek