[angstridd3n]

Hi there!
I usually know how to calculate the pH and pOH of certain solutions, but I came across this exercise, and I can't quite understand what I have to do.

The question says: "Determine the pH and pOH of a solution that results from mixing 15.00 ml HNO₃ 0.0500 M, 25.00 ml Ca(NO₃)₂ 0.0600 M and 10.00 ml KCl 0.020 M at the temperature of 25 ºC."

I can calculate the pH of HNO₃ but I can't grasp what can be done about the other two. I know that after obtaining the pH, it's easy to obtain the pOH (pOH= 14-pH, because we have a temperature of 25ºC).

I just want to understand how I can do it so I can have a go at it once more.
Sorry for bothering and thanks in advance! ^^

[Borek]

ionic strength and activity coefficients

[AWK]

Probably you have a problem with influence of ionic strength on activity of H3O+ and OH- ions.
You need an algorithm for activity coefficient calculations or you should take an aproximate values for activity coefficients from tables.
Calculate concentrations of all reagents in volume of 50 cm3.
HNO3 0.015M
Ca(NO3)2 0.03M
KCl 0.004 M
Total ionic strenth for them is 0.015 + 3x0.03 + 0.004 = 0.109 M ~ 0.1 M

From my table:
http://siorpc12.chem.pg.gda.pl/dyd/skrypt/TabelaDanych.pdf

f(H3O+) or f(OH-) for I=0.1 is equal to 0.7903 and pKw=13.801

Assuming HNO3 dissociate completely pH=-log(a_H3O+)=-log(f(H3O+)x[H3O+])=1.93
pOH=pKw-pH=13.801-1.926=11.87
instead of 1.82 and 12.18 , respectively, when ionic strength was neglected