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Analysis of Carbonate Compounds (38 Favorites)

LAB in Identifying an Unknown, Net Ionic Equation, Stoichiometry, Percent Composition, Molar Mass, Error Analysis. Last updated August 11, 2021.


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.

For the Student

Lesson

Purpose

In this lab you will be reacting carbonate compounds with hydrochloric acid to determine the amount of carbon dioxide that is released. Given that one mole of each of the compounds contains one mole of carbonate ions, this will allow you to calculate the mass of carbon in each compound. This, in turn, can be used to calculate the molar mass of each unknown compound. Remember that carbonates react with acids to form water, carbon dioxide, and a salt of the acid. You will be using excess hydrochloric acid, thus will be producing aqueous chloride salts.

Safety

You must wear aprons and safety goggles while performing this lab. Hydrochloric acid is very corrosive. If you spill any on your skin, wash it off immediately with water and notify the teacher. Avoid direct contact with the carbonate compounds and their solutions. If you get any of these on your skin, wash with soap and water.

Materials

  • Unknowns A, B, and C
  • 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

Procedure

  1. Obtain a 0.50-g sample of each of the unknown compounds. Describe the appearance of the compounds, making note of any differences among the three.
  2. Using test tubes and a stirring rod, test the solubility of each of the compounds in 10 ml of water. Indicate soluble or insoluble for each in the data table below.
  3. Use pH paper to measure the pH of the resulting solutions. Record these values in the data table below.
  4. Using a graduated cylinder, obtain 50 mL of 2M HCl and add this to a clean 125-mL flask. Find the mass of the flask plus the HCl to the nearest 0.01 g and record in the data table.
  5. Carefully measure approximately 4 g of the carbonate compound to be tested. Record the exact amount to the nearest 0.01 g.
  6. Very slowly add the solid carbonate compound to the flask containing the HCl. Do not add it all at once. Gently swirl the flask after adding each small amount to get a good mixture.
  7. After no additional CO2 bubbles are seen, find the mass of the remaining mixture and calculate the mass of CO2 lost.
  8. Pour the remaining solution down the sink with excess water. Rinse out the flask well. It is not necessary for it to be completely dried.
  9. Repeat steps 2-5 with the remaining unknown. Make sure that your lab station is left clean and that all glassware is cleaned and returned to its proper location.

Results/observation

Solubility of each solution: A ____________ B____________ C ____________

pH of each solution: A ____________B ____________ C____________

Unknown A

Unknown B

Unknown C

Mass of Sample

Mass of Flask + HCl

Total Mass of Sample,

Flask, and HCl

Final Mass

Mass of CO2 Released

Calculations and Analysis

  1. Use your data to calculate the molar mass of each of the carbonate compounds. (Remember that one mole of each compound contains only one mole of carbonate ions.) Please circle answers, and label each as A, B, or C.
  2. After being given the actual identities of the compounds, find your percent error in the molar mass of each compound. (% Error = |observed – actual| ÷ actual x 100%)
  3. What is one reason your calculated value for the molar mass of the unknown compounds might end up higher than the true value? What is one reason it might be lower than the true value? How could you prevent/limit these errors?
  4. Write the net ionic equations for each reaction of the carbonate compound plus the acid.
  5. Using the actual molar masses of the compounds, calculate the volume of CO2 released when 4.0 g of each of these reacts with excess HCl at 25º C and 1.00 atm pressure.
  6. Calculate the percentage of carbon in each of the three compounds.
  7. What would happen if a few drops of silver nitrate solution were added to one of your flask after excess hydrochloric acid has been added? Write the net ionic equation for this reaction.
  8. Why is it only necessary to wash out your flask, but not to completely dry it before testing the second and third compounds?