Question

Based on knowledge of solubility rules, would you expect Ksp of Na2S to be a large or a small number, why?

Answer 1

Na2S(s) <=> 2Na+ (aq.) + S-(aq.)

if Na2S has a solubility of 9.1 x 10^-8 M

ksp = [Na+]²[S-]
[Na+] = 2[Na2S] = 1.82x10^-7 M
[S-] = [Na2S] = 9.1x10^-8 M

ksp = (1.82x10^-7)²(9.1x10^-8) = 3.01x10^-21

Ksp is just a "specialized equilibrium" = [products]/[reactants] The equilibrium Ksp is usually used for slightly soluble products.

Ksp = Solubility Product

Since Na2S is slightly soluble in water it will only partially dissociate. And Thus an equilibrium willl be established

The Ksp express therefore should be [Na +] [S ] = Ksp

so of Na2S is very large number