Summary

In this lab, students calculate the molar masses of three unknown carbonate compounds by measuring the amount of product (CO₂) produced by a reaction with hydrochloric acid.

Grade Level

High School

AP Chemistry Curriculum Framework

This lab supports the following unit, topics and learning objectives:

• Unit 1: Atomic Structure and Properties
  • Topic 1.1: Moles and Molar Mass
    • SPQ-1.A: Calculate quantities of a substance or its relative number of particles using dimensional analysis and the mole concept.
  • Topic 1.3: Elemental Composition of Pure Substances
    • SPQ-2.A: Explain the quantitative relationship between the elemental composition by mass and the empirical formula of a pure substance.
  • Topic 1.4: Composition of Mixtures
    • SPQ-2.B: Explain the quantitative relationship between the elemental composition by mass and the composition of substances in a mixture.
• Unit 4: Chemical Reactions
  • Topic 4.2: Net Ionic Equations
    • TRA-1.B: Represent changes in matter with a balanced chemical or net ionic equation: a. For physical changes. b. For given information about the identity of the reactants and/or product. c. For ions in a given chemical reaction.
  • Topic 4.5: Stoichiometry
    • SPQ-4.A: Explain changes in the amounts of reactants and products based on the balanced reaction equation for a chemical process.

NGSS Alignment

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

• HS-PS1-7: Use mathematical representation to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.
• Scientific and Engineering Practices:
  • Using Mathematics and Computational Thinking
  • Analyzing and Interpreting Data

Objectives

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

• Determine the molar mass of a reactant from the mass of its product.
• Write a net ionic equation.

Chemistry Topics

This lab supports students’ understanding of:

• Stoichiometry
• Identifying an unknown
• Error analysis
  
• Net ionic equations
• Atomic mass
• Percent composition
• Ideal gas law
  

Time

Teacher Preparation: 20–30 minutes

Lesson: 40–60 minutes

Materials

For each group:

• Unknown A: Sodium carbonate
• Unknown B: Sodium bicarbonate
• Unknown C: Calcium carbonate (about 5 g each)
• pH paper
  
• 2-M HCl
• Test tubes (3)
• Stirring rod
• Erlenmeyer flask (125 ml)
• Graduated cylinder (50 mL or 100 mL)
• Weigh boats
• Scoopula
• Balance
• Safety goggles
• Lab aprons

Safety

• Students should wear aprons and safety goggles while performing this lab.
• Hydrochloric acid is very corrosive. If students spill any on their skin, they should notify you and wash it off immediately with water. Consult SDS for additional information.
• Avoid direct contact with the carbonate compounds and their solutions. If students get any of these on their skin, they should immediately wash with soap and water.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.

Teacher Notes

• In AP Chemistry, this lab can be used very early in the year during the study of stoichiometry. It helps to review several key concepts from first-year chemistry. In a first-year chemistry course, this lab could be used later in the year during the study of stoichiometry. In that case, students may need a bit more guidance on how to calculate the molar mass from the mass of CO₂ lost. Other questions (such as the ones involving the ideal gas law and net ionic equations) may need to be omitted or altered if those topics have not been covered yet.
• Students should not know the identities of the sodium carbonate, sodium bicarbonate, and calcium carbonate before beginning the lab – simply label them as Unknown A, Unknown B, and Unknown C, respectively.
• As an extension activity, you could discuss why the pH is different for the different carbonate compounds. For example, CaCO₃ is not very soluble in water, so it won’t affect the pH much at all, even though the addition of a soluble carbonate salt, like Na₂CO₃, will produce a basic solution. And NaHCO₃ produces a less basic solution (closer to neutral) than Na₂CO₃ because the bicarbonate ion already contains one hydrogen ion, so it will only remove a hydrogen from one water molecule to form carbonic acid, H₂CO₃, whereas Na₂CO₃ will remove hydrogens from two water molecules, leaving more OH^– and a more basic solution.