[Bacillus98]

Hello. I was wondering if someone could help me with solubility problems. I ended up taking a picture of my work via digital camera and I apologize if the pictures are too big at the link, it was the only way I could get them to be seen correctly. I kept getting the answer of
.00194 M.

The problem is: Find the number of moles of SrF2(s) that will dissolve in 1.00 L of a pH of 4.000. This to me is just the same as asking to calculate a solubility of a compound at a FIXED pH. I tried using the method my professor taught in class and but I cannot seem to get the book answer of 9.7 X 10^-4 M
The MB stands for mass balance and my professor said we should try to solve it via Ka constant substitutions. Charge balances do not apply apparently with FIXED pH problems.

http://img11.imageshack.us/img11/3210/img0404sg.jpg (top half)
http://img96.imageshack.us/img96/4057/img0405i.jpg    (bottom half)

Any help is greatly appreciated.

[Borek]

[F^-] + 1 + [H⁺]/Ka?

Solubility is number of moles of substance dissolved per volume - how is it related to Sr²⁺ concentration?

[Bacillus98]

I am not sure exactly. My professor had us use Ka to substitute in an equation and set it equal to S. We were getting the forms of S to put into the Ksp equation. Somehow this fits into the "systematic approach" method we are using to solve these problems.

[Borek]

How is number of moles of SrF₂ dissolved related to number of moles of Sr²⁺? It is not a rocket science, it is trivial question.

[Bacillus98]

I used a lecture example as a template to try to solve this problem. Perhaps this explains better...

This is how are lecture example was like, except there was not a number in front of the Cation.
LECTURE EXAMPLE: Calculate the solubility of calcium oxalate in a solution that has been buffered to pH=4.00
Lecture example ended up being:
After a bunch of tedious algebra, the following was obtained:

[Ca 2+]= [C2O4 2-]2.848     <--- this part was obtained by using the mass balance equation, then getting into terms of the ions that dissociated.
The number 2.848 was calculated by using Ka1 and Ka2 to get rid of unwanted unknowns. Then C2O4 was factored out. Ka1, Ka2, and the H+ concentrations were known, hence the number 2.848.
The professor then wrote:

S= [Ca2+]=2.848[C2O4]        Professor put in S for Ca 2+ and S/2.848 in for C2O4 into the Ksp expression.
Ksp= [Ca2+][C2O4]


Square root of (Ksp*2.848) = S


It's the "S" portion of calculation that I have trouble understanding what is really going on. That is why when you state "How is number of moles of SrF2 dissolved related to number of moles of Sr2+" I don't understand exactly since I am following the lecture example.

[Borek]

I am trying to start from the very beginning, please stop running ahead, just concentrate on what I am asking about.

What is solubility?

In the case of SrF₂, how is solubility related to the concentration of Sr²⁺?