[gobuckskb9]

I am just staring to learn about activity. I am trying to take a calculation using the Henderson-Hasselbalch equation with concentration and convert it to activity. I need to find the amount (grams) of sodium acetate that I need to add to a solution. I have the concentration of acetic acid, and need to find the activity (then concentration, then number of grams) of the conjugate base, acetate. I only know the concentration of the weak acid, and I supposed to use activity to solve the problem. So I need the activity coefficient which I am going to get using the Debye-Huckel equation. The conjugate base is acetate, and I can't seem to find the ion size (alpha) value anywhere. My equation has two unknowns, ion size (alpha) and Ionic strength (mew). I need to solve for the coefficient so I can solve for the activity acetic acid so I can solve for the activity of acetate.
I think I am missing something important about manipulating these equations, if someone could point that out I would really appreciate it.

[gobuckskb9]

I would like to add that I do know how to solve for the ion strength, so I only need to figure out what this alpha term is doing in the equation and where I can find the value.

[Borek]

There are several versions of Debye-Huckel formula for activity coefficients. Common approach when you don't know size of the ion is to assume that product B*α equals 1 (it is not far from the reality). Then you can use equation in the form listed here:

http://www.chembuddy.com/?left=pH-calculation&right=ionic-strength-activity-coefficients#eq17.2