Besides temperature, ionic strength, and common ion effect, the pH of a solution can also have a large impact on solubility whenever one (or both) of the dissociated ions from a solid can act as a weak acid/base. Consider the series of simultaneous equilibria shown below (you can assume all species are aqueous unless otherwise noted): CaF2(s) ⇌ Ca2++ 2F-
Ca2++ F-⇌ CaF+
CaF2(s) ⇌ CaF2(aq)
Ca2++ H2O ⇌ CaOH++ H+
F -+ H+ ⇌ HF
HF + F- ⇌ HF2-
H2O ⇌ H+ + OH-
(a) Write the charge-balance expression for this system.
(b) Write the mass-balance expressions for this system.

answer: - first one is the correct answer. .

explanations: - potassium sulfide is and cobalt(ii) chloride is .

charge on k is +1, charge on s is -2, charge on co is +2 and charge on cl is -1.

in a double replacement reaction the ion exchange takes place. in general the reaction looks like:

if one of the product is insoluble as per solubility rules then a precipitate of it is formed.

when we mix the aqueous solutions of potassium sulfide and cobalt(ii) chloride then chloride ion goes with potassium ion and sulfide ion goes with cobalt ion and the balanced equation is written as:

k and cl has equal charges, similarly co and s has equal charges and hence no subscripts are used for the product side.

as per the solubility rules, cos is insoluble and so it's precipitate is formed.

looking at the given choices, first choice is correct.