Question

A mineral sample is analyzed for its cobalt and calcium content. A sample is dissolved, and then the cobalt and calcium are precipitated as Co(OH)2(s) and Ca(OH)2(s). At what pH can Co(OH)2(s) be separated from Ca(OH)2(s) at 25 °C? Assume that an effective separation requires a maximum concentration of the less soluble hydroxide of 1× 10^–6 M.

Please help! So far I have gotten 12.46 as the PH and .0291 as the concentration but it's wrong! Help ! I'm super confused.... Thank you in advance!!

Answer 1

If we are to obtain the same answers we should have the Ksp values you're working with.

Answer 2

These are the given Ksp values to use:

Ksp of Co(OH)2 is 5.9x10^-15 M^3
Ksp of Ca(OH)2 is 5.0x10^-6 M^3

Answer 3

I would do this.
Ksp Co(OH)2 = 5.9E-15 so
(OH^-) = sqrt(Ksp/1E-6)
Then convert OH to pH. I get an answer of something like 9.9 or so.

Answer 4

Thanks Dr.Bob!! I was then able to plug into the second equation and found the concentration of calcium! You were great help! Thanks again :)