Question 22

[jimmytech3124]

Hi guys,i am having trouble with this question below. I am having trouble understanding what it means by just "begin" the precipitation (i know what precipitation means). The solution uses ksp of ZnS and finds the concentration of S^2- required to begin percipitation. I would really appriciate if somone could help. Thanks in advance

If in a solution containing only Zn2+, the concentration of zinc ions is 5.0 x 10-3 M, what concentration of S2- is required to just begin the precipitation of zinc sulfide?

5.0 x 10-24 M[x]
2.0 x 10-19 M[x]
3.2 x 10-11 M[x]
5.0 x 10-3 M[x]

[admin]

To begin with, there is no difference with the concentration required to begin precipitation and the minimum concentration required to produce a precipitation. The key idea is that the solubility product equilibrium constant Ksp allows one to calculate the maximum amount of product which is soluble. If you add one atom more then it results in a precipitate. Thus you can calculate the relevant concentration using the Ksp expression as described in the Explanation (which is cross-referenced to The Gold Standard MCAT text or you can read further using the link below):


http://dbhs.wvusd.k12.ca.us/webdocs/Equilibrium/Writing-Ksp-expression.html

[jimmytech3124]

Thank you for the explanation, i appriciate your help

[starter15]

I answered D because I thought the concentrations where 1:1 being that that's what the equilibrium expression for dissociation showed. Guess not.

I'll just always use the Ksp values and always know they are for all atom combinations of salts of that particular kind.