What is the equilibrium equation for the dissociation of formic acid in water? HCOOH (aq) + H2O (l) = H3O+ (aq) + HCOO- (aq) A) Ka = [H3O+][HCOO-]/[HCOOH] B) Ka = [H3O+][HCOO-]/[H2O] C) Ka = [HCOO-][HCOOH]/[H2O] D) Ka = [H3O+][HCOO-]/[HCOO-]
The Correct Answer and Explanation is:
The correct equilibrium equation for the dissociation of formic acid (HCOOH) in water is:
A) Ka = [H3O+][HCOO-]/[HCOOH]
Explanation:
Formic acid (HCOOH) is a weak acid, and when it dissolves in water, it undergoes partial dissociation into hydronium ions (H3O+) and formate ions (HCOO-). The dissociation process is represented by the following equilibrium equation:
HCOOH (aq) + H2O (l) ⇌ H3O+ (aq) + HCOO- (aq)
This reaction is at equilibrium, and the extent to which formic acid dissociates in water is governed by the acid dissociation constant (Ka). The Ka expression is derived from the concentrations of the species involved at equilibrium.
The acid dissociation constant, Ka, is given by the ratio of the concentrations of the products (H3O+ and HCOO-) to the concentration of the undissociated formic acid (HCOOH). This is because the concentration of water (H2O) is considered constant in dilute solutions, as it is the solvent and its concentration remains approximately 55.5 M. Therefore, the concentration of water does not appear in the Ka expression for weak acids in aqueous solutions.
The correct Ka expression is:
Ka = [H3O+][HCOO-] / [HCOOH]
This equation accurately represents the dissociation of formic acid in water, where the concentrations of H3O+, HCOO-, and HCOOH are used to calculate the acid dissociation constant. The concentration of H2O is not included because it is assumed to be constant and does not significantly change during the dissociation process.
In summary, option A is the correct expression for the acid dissociation constant of formic acid.
