The following pictures represent initial concentrations in solutions that contain a weak acid HA (pKa = 6.0) and its sodium salt NaA. (Na+ ions and solvent water molecules have been omitted for clarity.)
. (c) Draw a picture that represents the equilibrium state of solution (1) after the addition of two OH-ions.

Verified step by step guidance

1. Identify the initial components in solution (1): HA (weak acid) and A^- (conjugate base).
2. Recognize that adding OH^- ions will react with HA to form A^- and water, according to the reaction: HA + OH^- \rightarrow A^- + H_2O.
3. Since two OH^- ions are added, two HA molecules will be converted to two A^- ions and two water molecules.
4. Draw the new equilibrium state: decrease the number of HA molecules by two and increase the number of A^- ions by two, while adding two water molecules.
5. Ensure the total number of particles remains constant, and the solution reflects the equilibrium after the reaction with OH^- ions.

Key Concepts

Weak Acids and Their Equilibria

Weak acids, like HA, do not completely dissociate in solution. Instead, they establish an equilibrium between the undissociated acid (HA) and its ions (H+ and A-). The position of this equilibrium is influenced by the acid's pKa value, which indicates the strength of the acid; a higher pKa means a weaker acid. Understanding this equilibrium is crucial for predicting how the addition of substances, such as OH- ions, will affect the concentrations of HA and A-.

Buffer Solutions

A buffer solution consists of a weak acid and its conjugate base, which helps maintain a stable pH when small amounts of acids or bases are added. In this case, the presence of HA and its salt NaA allows the solution to resist changes in pH upon the addition of OH- ions. The buffer action occurs through the equilibrium between HA and A-, which can react with added OH- to minimize pH changes.

Reaction of Hydroxide Ions with Weak Acids

When hydroxide ions (OH-) are added to a solution containing a weak acid, they can react with the hydrogen ions (H+) produced by the acid. This reaction shifts the equilibrium towards the formation of more A- ions, as the added OH- effectively removes H+ from the solution. Understanding this reaction is essential for predicting the changes in the concentrations of HA and A- after the addition of OH- ions.