Hello,
I am a bit frustrated because i have this problem
A solution containing 0.100 M Acetate buffer and 1.00*10^-4 M NiSO4 has a pH 5.00. What is the concentration of [Ni2+] for this solution?
Step 1) Write a mass balance equation for nickel ion (NiTotal) where the two tupes of ions are Ni2+ and the nickel-acetate complex ion, NiOAC+.
Now, im thinking to myself there are 2 sources of nickel here (The 1.^-4 M NiSO4), but i have to account for acetate as well right?
So my charge balance would be
[Ni2+] + [NiOAc+]= [NiSO4] + [OAc]
Step 2) Write a mass balance equation for acetate (OAc Total) where there are three forms of acetate: acetate ion, acetic acid, and NiOAc+.
I wrote
OAcT= [HOAc] + [ OAc-] + [ NiOAc+]
Then i thought why i am using OAcT cant there be anything else? and there isnt really that i can think of.
Step 3) Write the Expression for Kf NiOAc+
I wrote
Ni2+ + OAc-
NiOAc+
Kf= [NiOAc+]/[Ni2+] [OAc-]
Step 4) Write the expression for Ka of acetic acid
HOAc
H+ +OAc-
Ka=[HOAc]/[H+] [OAc-]
In steps 1-4 you developed 4 equations. Verify that there are four unknowns ([Ni2+], [HOAc], [OAc-] and [NiOAc+]) in this system of equations. H+ is not an unknown for this problem because the pH was given in the problem. The goal is to solve this system of equations for [Ni2+] so that we have an equatuion with only known values (kf, ka, OAcTotal, and NiTotal). The first step is to reaarrange the expression for Kf so that the term NiOAc+ is isolated.
i did so
[NiOAc+]=[OAc-] + [Ni2+] + Kf
Next substitute the equation you wrote in question 9 into the mass balance for nickel. Rearrange this equation so that the term Ni2+ is isolated on one side of the equation.
I did this
But here is where i get in trouble
[Ni2+]= [NiOAc+] + [OAc-] + [NiSO4]
As you can see, from the equation derived above, you have an expression for Ni2+ that has only one unknown ([OAc-]).
But this is not the case
i dont know what im doing wrong.