[Turk]

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.

[Borek]

Step 1) Write a mass balance equation for nickel ion

So my charge balance would be

So mass, or charge?