[SHIVANGI]

I have to calculate the pH of citric acid(0.1M) and sodium citrate(0.1M).I calculated the pH from the charged balance equation and found it to be 3.105. However while doing the calculations I have assumed the activity coefficients to be equal to one.Now from the pH which I have calculated I get concentration of all species. Now using these concentrations I calculate the ionic strenght and then the activity coefficients, I do not take into account OH since it is negligible at this pH value. After getting the new activity coefficients do I again use them in charge balance equation, recalculate all concentrations and the pH and continue the iterations till the value of ionic strenght and hence pH doesnt change?

[Borek]

Yes, this has to be done iteratively.

[SHIVANGI]

I tried doing it by iteration but I feel I am going wrong, Assuming both H2A and HA to be negligible in proton balance equation and using the equation for the first dissociation (7.41E-04)
constant i got an quadratic equation which is
H+^2+0.051H -6.03^10-5=0
I got an H value of 5.9^10-5 which gives me a pH value of 4.2
The ionic strenght calculated was found to be 0.103
and the activity coefficient used to obtain above equation were


γNa           0.768
 γC6H7O7   0.760
γC6H6O7   0.361
γC6H5O7   0.110
H           0.825
Am I doing it correctly

[Borek]

All I can do at this stage is to confirm you are right about being wrong. Your pH is more than 1 unit too high (should be slightly below 3).