Question

ZnCO₃, a slightly soluble substance, is least soluble in 0.20M ZnCl₂ or 0.10M Na₂CO₃.

Please explain how to work this out

Answer 1

The ZnCO3 will get solublise by the following reaction,

ZnCO3(s) <--------> Zn2+(aq) + CO32-(aq)
Now, in the case of ZnCl2 - Zn2+ is the common ion and according to Le Chatelier's principle it will shift the equlibrium to the left side leading to the decreased solubility.

Now, in the case of Na2CO3 - CO3^2- is the common ion and according to Le Chatelier's principle it will shift the equlibrium to the left side leading to the decreased solubility again.

But 0.20 M ZnCl2 gives 0.20 M Zn2+ ions in comparison to 0.10M Na2CO3 which gives only 0.10M CO3^2- common ions, therefore the equlibrium will get shift more left in the case of 0.20M ZnCl2 leading to the least solubility.

ZnCO3 will be least soluble in 0.20M ZnCl2.