[99199387]

My problem is how to make 2.00 liters of a buffer containing 0.150 M NaCl and 2.0 mM CaCl2.at pH = 8.40. The buffer should maintain the pH within a 0.5 pH unit range even if 0.020 moles of a strong acid or base is added to your buffer. This is the list of compounds to choose from (http://i.imgur.com/x8gvI2h.png)
I'm not exactly sure on how to start the calculations. I was thinking I would need to use the henderson hasselbach equation but I am a little stuck.

[Borek]

See if these don't help:

http://www.chembuddy.com/?left=buffers&right=composition-calculation

http://www.chembuddy.com/?left=buffers&right=pH-change

[99199387]

So I would probably pick tris Hcl since the pka is near the desired pH, but I am a little confused as to how to use the henderson hasselbach equation when theres CaCl2 and NaCl in solution as well.

Do I have to do somethign with finding the ionic strength?

[Borek]

Do I have to do somethign with finding the ionic strength?

Yes. This will be slightly tricky, as you are outside the Debye-Huckel theory applicability range, but you can still try to use Davies equation.

[99199387]

so would it be like pH = pKa + [A-]activity coeff / [HA]activity coeff

I'm not sure which activity coefficient  would be on top and bottom

but more importantly I am uncertain on how to use the Debye-Hückel equation.
I found ionic strength using I=1/2 sum(con*charge^2), but in the equation I'm not sure what the charge would be since I have two ions in my compound and also not sure what atomic size i would use. Since they arent charged compounds im not even sure if im going in the right direction.

[Borek]

There will be different activity coefficient for each ion, depending on what charge it has. We assume if the substance is not charged its activity coefficient equals 1.

While the most exact form of DH equation uses atomic sizes, there are several less exact formulations of the theory that use some simplifying assumptions. Compare http://www.chembuddy.com/?left=pH-calculation&right=ionic-strength-activity-coefficients