Problem: What is the minimum S2− concentration that will cause ZnS to start precipitating from a 0.10 M solution of Zn(NO3)2, a very soluble salt? Ksp for ZnS is 1.1 × 10−21.

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For this problem, we’re being asked to calculate the concentration of S2– needed in order to precipitate ZnS from a 0.10 M Zn(NO3)2 solution.


Since the compounds are ionic compounds, they form ions when dissociating in water. The dissociation of ZnS and Zn(NO3)2 in water are as follows:


The sulfide ion, S2–, has a charge of –2. Zinc then has a charge of +2:

ZnS(s)  Zn2+(aq) + S2–(aq)


The nitrate ion, NO3, has a charge of –1. Zinc then has a charge of +2:

Zn(NO3)2  Zn2+(aq) + 2 NO3(aq)


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What is the minimum S2− concentration that will cause ZnS to start precipitating from a 0.10 M solution of Zn(NO3)2, a very soluble salt? Ksp for ZnS is 1.1 × 10−21.

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