A 20.0-mL sample of 0.125 M HNO₃ is titrated with 0.150 M NaOH. Calculate the pH for at least five different points throughout the titration curve and sketch the curve. Indicate the volume at the equivalence point on your graph.

Verified step by step guidance

1. Calculate the initial pH of the 0.125 M HNO3 solution using the formula: pH = -log[H+].
2. Determine the volume of NaOH needed to reach the equivalence point using the formula: (M1 * V1) = (M2 * V2), where M1 and V1 are the molarity and volume of HNO3, and M2 and V2 are the molarity and volume of NaOH.
3. Calculate the pH after adding a small volume of NaOH (e.g., 5 mL) by determining the moles of HNO3 and NaOH, finding the limiting reagent, and using the remaining concentration of H+ to find the pH.
4. Calculate the pH at the equivalence point by recognizing that the solution will be neutral (pH = 7) because HNO3 and NaOH are a strong acid and strong base, respectively.
5. Calculate the pH after adding excess NaOH (e.g., 5 mL beyond the equivalence point) by determining the concentration of excess OH- and using the formula: pOH = -log[OH-], then converting to pH using: pH = 14 - pOH.

Key Concepts

Titration

Titration is a quantitative analytical method 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 the reaction reaches its equivalence point, where the amount of titrant equals the amount of substance in the sample. The pH changes during this process, allowing for the calculation of the unknown concentration.

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 present in the sample solution. For strong acid-strong base titrations, this point typically occurs at a pH of 7. Understanding the equivalence point is crucial for accurately determining the pH at various stages of the titration curve.

pH Scale

The pH scale measures the acidity or basicity of a solution, ranging from 0 (very acidic) to 14 (very basic), with 7 being neutral. The pH is logarithmically related to the concentration of hydrogen ions in the solution. During a titration, the pH changes significantly, especially near the equivalence point, making it essential to calculate and plot pH values at various volumes of titrant added to understand the titration curve.