Experiment 11: pH Measurements and Indicators for Acid–Base Titrations for AP Chemistry

For a quick review on equilibrium, refer to the following concepts:

• Le Chatelier's Principle for AP Chemistry
• Acid–Base Equilibrium for AP Chemistry
• Ka, Kw, Kb - The Acid, Water, and Base Dissociation Constant for AP Chemistry
• Buffers for AP Chemistry
• Titration Equilibria for AP Chemistry
• Solubility Equilibria for AP Chemistry

Synopsis

The acidity of various substances is determined with a pH meter or acid–base indicators. This may also be done by mixing or diluting solutions.

Equipment

analytical balance
Erlenmeyer flask
graduated cylinder
pH meter
pipet
volumetric flask

Measurements

1. Weigh each acid or base into a volumetric flask, and dilute to volume.
2. Pipet a sample of an acid or base of known concentration into a volumetric flask, and dilute to volume.
3. Pipet dififferent solutions into a flask.
4. Measure the pH of the solutions using a pH meter.

Calculations

All weighed samples are converted to moles by using the molar mass, and the moles are divided by the volume of the volumetric flask in liters to yield molarity.

The concentrations of the diluted solutions (measurement 2) are calculated by using the dilution equation.

The concentrations of the other solutions (measurement 3) are calculated from the balanced chemical equation and the dilution equation.

The pH may be estimated by adding an acid–base indicator to any of the prepared solutions.

The hydrogen ion concentration, hydroxide ion concentration, or pOH may be calculated from the pH. One or more of these may be used to determine the concentration of all other species in the solution.

Comments

The original acids and bases may all be solids or solutions, or a mixture of both.