Write an expression for the acid ionization constant (Ka) for the dissociation of the following weak acid H2CO3.

Question

BlackTom AI · Accepted Answer

Write the ionization as a chemical equilibrium and apply the definition of Ka. For the first proton loss of carbonic acid the reaction is:
H2CO3 + H2O ⇌ H3O+ + HCO3−

Relevant knowledge:
- The acid dissociation constant Ka is defined for HA + H2O ⇌ H3O+ + A− as Ka = [H3O+][A−]/[HA].
- Pure liquids (like H2O) are omitted from the equilibrium expression; concentrations are expressed as [ ] (molarity) or activities.
- Carbonic acid is diprotic, but this expression applies to the first dissociation (forming HCO3−).

Step-by-step reasoning and derivation:
1. Write the balanced equilibrium: H2CO3 + H2O ⇌ H3O+ + HCO3−.
2. Form the equilibrium-constant expression including product concentrations in the numerator and reactant concentrations in the denominator, excluding pure liquid water:
   Ka = ([H3O+][HCO3−]) / ([H2CO3][H2O]) but omit [H2O] as it is a pure liquid.
3. Simplify to:
   Ka = [H3O+][HCO3−] / [H2CO3].

Why this is correct:
- It follows the general Ka form and includes both products (H3O+ and HCO3−) and the undissociated acid (H2CO3) in the denominator. Omitting water is correct because water is the solvent and its activity is effectively constant.

Why other options are incorrect (brief):
- Ka = [HCO3−]/[H2CO3]: Missing [H3O+] (omits one product).
- Ka = [H3O+]/[H2CO3]: Missing [HCO3−].
- Ka = [H2CO3]/([H3O+][HCO3−]): Inverse of the correct expression.
- Ka = [H2CO3]/[HCO3−] or similar: Wrong placement of species.

Conclusion:
The correct acid dissociation expression is Ka = [H3O+][HCO3−] / [H2CO3], which matches the provided answer.

Write an expression for the acid ionization constant (Ka) for the dissociation of the following weak acid H2CO3.单项选择题

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