The following pictures represent solutions at various stages in the titration of sulfuric acid H2A (A2- = SO4 2-) with aqueous NaOH. (Na+ ions and water molecules have been omitted for clarity.)

. (a) To which of the following stages do solutions 1–4 correspond? (i) Halfway to the first equivalence point (ii) Halfway between the first and second equivalence points (iii) At the second equivalence point (iv) Beyond the second equivalence point

Verified step by step guidance

1. Identify the species present in each solution: H2A, HA-, and A2-.
2. Determine the stage of titration based on the presence of these species.
3. For solution 1, observe the presence of H2A and HA- ions, indicating it is halfway to the first equivalence point.
4. For solution 2, observe the presence of HA- and A2- ions, indicating it is halfway between the first and second equivalence points.
5. For solution 3, observe the presence of A2- ions, indicating it is at the second equivalence point.

Key Concepts

Titration

Titration is a quantitative analytical technique used to determine the concentration of a solute in a solution. It involves the gradual addition of a titrant (a solution of known concentration) to a sample solution until a reaction reaches its endpoint, often indicated by a color change. In this case, sulfuric acid (H2A) is being titrated with sodium hydroxide (NaOH), which is a strong base.

Equivalence Point

The equivalence point in a titration is the stage at which the amount of titrant added is stoichiometrically equivalent to the amount of substance in the sample solution. For sulfuric acid titrated with NaOH, there are two equivalence points due to the diprotic nature of sulfuric acid, meaning it can donate two protons (H+ ions) during the reaction. Understanding the equivalence points is crucial for determining the stages of the titration.

Halfway to Equivalence Point

The halfway to the equivalence point refers to the stage in a titration where half of the acid has been neutralized by the base. At this point, the concentrations of the weak acid and its conjugate base are equal, which is significant for calculating pH and understanding buffer systems. In the context of the titration of sulfuric acid, identifying this stage helps in predicting the pH changes and the behavior of the solution as titration progresses.