Grade Level

In this activity, students will interact with a simulation to practice calculating values from titration data. AP level students select an analyte from a list of weak and strong acids and bases. A pH vs. volume curve is generated using randomized values for the titration, and students use the trace function on the graph to identify the appropriate data points for calculating the concentration and, for weak acids or bases, the value of Ka or Kb, for the selected analyte. The simulation is not designed to be used in place of the lab technique, but rather as a way of strengthening the skills already learned in the lab.

Grade Level

High School, AP Chemistry

AP Chemistry Curriculum Framework

This simulation activity supports the following units, topics, and learning objectives:

• Unit 8: Acids and Bases
  • Topic 8.5: Acid-Base Titrations
    • 8.5.A: Explain results from the titration of a mono- or polyprotic acid or base solution, in relation to the properties of the solution and its components.
  • Topic 8.7: pH and pKa
    • 8.7.A: Explain the relationship between the predominant form of a weak acid or base in solution at a given pH and the pKa of the conjugate acid or the pKb of the conjugate base.

NGSS Alignment

This simulation activity will help prepare your students to meet the performance expectations in the following standards:

• HS-PS1-7: Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.
• Scientific and Engineering Practices:
  • Analyzing and Interpreting Data
  • Planning and Carrying Out Investigations

Objectives

By the end of this simulation activity, students should be able to:

• Use titration data to
  • calculate the concentration of an acid or a base
  • determine the equivalence point and half-equivalence point for the titration
  • calculate the value of the dissociation constant (K_a or K_b) for a weak acid or weak base

Chemistry Topics

This simulation activity supports students’ understanding of:

• Acids & Bases
• Titrations
• Strong vs. weak acids/bases
• Acid base reactions
• pH
• Equivalent Point
• Equilibrium constants

Time

Teacher Preparation: 5 minutes (photocopy student handout)

Lesson: 30-50 minutes

Materials

• Student Handout
• Device connected to the internet
• Computer access to https://teachchemistry.org/classroom-resources/acid-base-titration

Safety

• There are no particular safety hazards in this activity.

Teacher Notes

• This simulation can be found at the following link (note that students can access the simulation without an AACT login): https://teachchemistry.org/classroom-resources/acid-base-titration
• Teachers can use the simulation to best meet their needs. For example, it can be integrated into a lecture or review, without using the available student handout, or alternatively, teachers can adapt the student handout, or create their own, to be used in any phase of learning, or for formative assessment.
  • This AP version of the student activity is intended for use in AP Chemistry classes. Some portions could also be used with advanced first-year classes.
  • An alternative general chemistry high school version of this activity, Acid-Base Titrations, is also available.
  • This activity contains prompts to remind students how to determine the values needed to calculate concentrations for each of four known solutions, along with the K_a or K_b for the weak acid and weak base options. The simulation also contains an unknown weak acid and an unknown weak base that will have randomly generated K_a or K_b values that can be used many times over for practice.
  • As with titrations in the lab, the data points may not perfectly line up with the ideal points on the graph. The simulation allows for some tolerance and students should choose the data point that is closest to the ideal value. In the image below, the red arrow shows the equivalence point. The graph trace, shown with a label, identifies 13.29 mL as the volume of the closest data point to the equivalence point. The next closest volume is 13.28 mL. Students can use either of these volumes correctly in their calculations, and in finding the pH at the half-equivalence point, will get a correct concentration value and a correct value for K_a.

• This activity is designed to be used as a simple practice activity, but it has opportunities for many other types of lessons.

• Student pre-requisite skills and knowledge (with links to related resources):
  • Writing and balancing equations for reactions of acids with bases:
    • Acid Base Reactions is an introduction to the chemistry of acids and bases.
  • Use of the mole ratio and stoichiometry:
    • Acid/Base Stoichiometry allows students to explore stoichiometry by collecting carbon dioxide generated from an acid-base reaction.
  • Lab experience with any form of a titration process, even without the standard equipment (such as microscale processes and dropwise analysis):
    • Titration Curves uses drops from a plastic pipet to teach the titration curve without using a buret.
  • Brønsted-Lowry theory of acids and bases:
    • Categorizing, Calculating and Applying Concepts from Weak Acids, Weak Bases and Salts is a lesson plan that addresses this theory.
  • Weak acid and weak base dissociation equations and the corresponding equilibrium constants (K_a and K_b):
    • Titration addresses differences between strong and weak acids and prompts students to consider how titration data for the two types might differ.
• This simulation should be used to reinforce titration skills, rather than to replace the lab experiment.
  • AP students can use the simulation to learn how the behaviors of weak acids and bases differ from those of strong acids and bases, and to translate between mathematical models (pH curve) and symbolic models (net ionic equations) when analyzing acid-base reactions.
  • Teachers can also add a particulate level to the activity by having students analyze and draw particle models to represent how the contents of the flask change throughout the titration process.
• The simulation can be used as many times as needed for additional practice. Each time the simulation is reset, new randomized values are generated.
• Titrant settings (in the buret):
• Solution
• Either HCl or NaOH is automatically selected, based on student selection of analyte.

• Concentration
  • When analyte is a strong acid or base, students will select a concentration for the titrant from a pull-down menu.
  • When analyte is a weak acid or base, the program will automatically generate a concentration value for the titrant and display it on the screen.

• Analyte solution (in the flask) – selected by student:
  • Hydrochloric acid, HCl – strong acid
  • Sodium hydroxide, NaOH – strong base
  • Acetic acid, HC₂H₃O₂ – weak acid with K_a = 1.75 x 10^-5
  • Ammonia, NH₃ – weak base with K_b = 1.76 x 10^-5
  • HA, unknown weak acid with K_a randomly generated
  • B, unknown weak base with K_b randomly generated

• There is a Reset button at the top right of the screen. Pushing this button will bring the user back to the beginning screen to choose a new substance. With each reset, the random value generation begins anew.
• Students should check answers by entering their values in the answer boxes.
  • If submitted answers are incorrect, hints will be displayed. The students should check their work and re-calculate to see if they can correct their mistakes. To enter a new value, students must push Dismiss after reading the hints. After the second answer, the correct values are displayed. (Student will not be told whether they entered the correct answer at this point, as they can check their answers based on the display.)
  • There is a Reset button under the titration set-up image at the left of the screen and underneath the answer boxes where students check their answers. Pushing this button will bring the user back to the beginning screen to choose a new substance. With each reset, the random value generation begins anew.

• Note: The simulation is programmed based on mathematical models that are typically taught at the high school and AP levels. As in most texts used at this level:
  • The contribution of hydrogen ions from the autoionization of water is ignored. (No K-values lesser than 10^-6 are included.)
  • Concentrations are used, rather than activities.
  • As with laboratory-based simulations, there may not be a data point at the exact spots where a student should look, but the “correct” answers include a tolerance to allow for estimates that fall between the data increments.
  • Approximations are made at certain points across the graph to ensure the equivalence and half-equivalence points are as close as possible to the designated values, and that the initial solution pH is correct at the 0-mL mark as calculated using, for example, K_a = x²/[HA]₀, with [H₃O⁺]=x, then pH=-log[H₃O⁺].