[sainte]

Assume you have a 1.0 mM solution of Na3PO4 and a 1.0 mM solution of NaH2PO4. What are the pHs of these solutions if they are in equilibrium with atmospheric CO2? How much of each solution would you need to mix in order to make 500 mL of phosphate buffer having a pH of 7.0.

visual-minteq can be used to provide other details for this
http://www.lwr.kth.se/english/OurSoftWare/Vminteq/

but i am sure it can be worked out without it too.

all help is mucho appreciated.

[sainte]

if someone can just get me started on this would help me a lot .

[Borek]

First question requires information about partial pressure of atmospheric CO₂, Henry's constant for CO₂, and two equilibria - carbon dioxide reaction with water to create carbinc acid, and carbonic acid dissociation. These equilibria are quite often combined into one, sometimes together with carbon dioxide dissolution.

Does the second question requires you to use CO₂ saturated solutions, or just solutions as given? If carbon dixoide is to be ignored that is rather simple buffer question, if not - you will most likely have to deal with rather complex system.

[sainte]

yes CO2 has to be added into it , i dont know if u have ever used visual minteq ,

the program lets you add ions as seperate components into the software like if i wanted to enter

Na3PO4 , it will be entered as Na+ = 3.0mM , H+ = 1.0mM , PO4-3 = 1.0mM

then i run the program and it gives me a pH of just this solution in water as 8.230 , ofcourse in gases you add CO2 as a component aswell.

then i do the same for NaH2PO4 and i get 5.092

now what i have are conc. of H+ and PO4-3 and Na+ to tweak in the prog to get a ph = 7 from those conc.

and thats where i am having a problem , thx.

[Borek]

i dont know if u have ever used visual minteq

I am not using equilibrium programs, I am writing them

Na3PO4 , it will be entered as Na+ = 3.0mM , H+ = 1.0mM , PO4-3 = 1.0mM

Recheck what you wrote.

then i run the program and it gives me a pH of just this solution in water as 8.230 , ofcourse in gases you add CO2 as a component aswell.

then i do the same for NaH2PO4 and i get 5.092

now what i have are conc. of H+ and PO4-3 and Na+ to tweak in the prog to get a ph = 7 from those conc.

and thats where i am having a problem , thx.

No idea how minteq works, but I would be not suprised if it will work OK if you start with H₃PO₄ and NaOH, this is equivalent to PO₄^3- and Na⁺ mixture. Try to find (by trial and error) such ratio of NaOH and H₃PO₄ that pH is 7.00, then calculate how to mix Na₃PO₄ and NaH₂PO₄ to get identical solution.