Final answer:
The mass balance for a 0.05 M K2S solution considers the full dissociation of K2S into K+ and S2- ions and the weak base behavior of S2- reacting with water to form HS- and OH-. The equations encapsulate the stoichiometric ratios and account for all species resulting from these reactions in solution.
Step-by-step explanation:
The mass balance equation for a solution of 0.05 M fully dissociated K2S, noting that S2- is also a weak base, involves accounting for the dissociation of K2S into its ions and the subsequent reaction of S2- with water. Because K2S is a salt composed of a strong electrolyte (K+) and a weak base (S2-), its dissolution in water can be represented by:
K2S (s) → 2K+ (aq) + S2- (aq)
For every mole of K2S that dissociates, two moles of K+ ions and one mole of S2- ions are produced.
However, since S2- is a weak base, it can react with water in a typical acid-base reaction:
S2- (aq) + H2O (l) ⇌ HS− (aq) + OH− (aq)
This secondary reaction must be considered in the mass balance.
The mass balance equation thus accounts for all species in solution:
[K+] = 2[M]
[S2-] + [HS−] = [M]
[OH−] ≈ [S2-] (if we assume full conversion to HS− for simplicity)
Where [M] represents the initial molarity of the K2S solution, which in this case is 0.05 M.