Mercury (I) carbonate, Hg2CO3 is a salt of low solubility. When placed in water, it dissolves until an equilibrium is reached [the mercury (I) ion, Hg22+, is diatomic]:
Hg2CO3(s) → Hg22+ (aq) + CO32- (aq)
Write an expression for the solubility product constant, ksp, of Mercury (I) carbonate.
The Correct Answer and Explanation is:
Solubility Product Expression for Mercury(I) Carbonate (Hg₂CO₃)
The solubility product constant (KspK_{sp}) expression for the dissociation of Mercury(I) carbonate is: Ksp=[Hg22+][CO32−]K_{sp} = [\text{Hg}_2^{2+}] [\text{CO}_3^{2-}]
Explanation
Mercury(I) carbonate, with the formula Hg2CO3\text{Hg}_2\text{CO}_3, is an ionic salt that has low solubility in water. When it is added to water, only a small amount dissolves before establishing a dynamic equilibrium between the solid salt and its dissociated ions in solution.
The dissolution reaction is: Hg2CO3(s)⇌Hg22+(aq)+CO32−(aq)\text{Hg}_2\text{CO}_3 (s) \rightleftharpoons \text{Hg}_2^{2+} (aq) + \text{CO}_3^{2-} (aq)
Here, Hg₂²⁺ is a diatomic cation, meaning it consists of two mercury(I) atoms bonded together. This is characteristic of mercury(I) compounds, as the mercury(I) ion (Hg⁺) is unstable alone and always forms the diatomic ion Hg22+\text{Hg}_2^{2+}.
The solubility product constant (Ksp) is an equilibrium constant used for sparingly soluble salts. It quantifies the product of the molar concentrations of the dissociated ions, each raised to the power of their stoichiometric coefficients from the balanced dissolution equation. Since there is one mole of Hg₂²⁺ and one mole of CO₃²⁻ produced for every mole of Hg₂CO₃ that dissolves, the exponents in the expression are both 1.
Thus, the KspK_{sp} expression is: Ksp=[Hg22+][CO32−]K_{sp} = [\text{Hg}_2^{2+}] [\text{CO}_3^{2-}]
This expression allows us to calculate or understand the extent to which Hg₂CO₃ can dissolve in water under equilibrium conditions. Since it’s a low-solubility compound, the concentrations of the ions will be small, and so will the value of KspK_{sp}. Note that solids are not included in the equilibrium expression, which is why Hg2CO3(s)\text{Hg}_2\text{CO}_3(s) does not appear in the formula.
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