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Analysis of Carbonate Compounds (9 Favorites)

LAB in Identifying an Unknown, Net Ionic Equation, Percent Composition, Atomic Mass. Last updated April 25, 2019.


Summary

In this lab, students identify three unknown substances by measuring the amount of product (CO2) produced by a reaction with hydrochloric acid.

Grade Level

High school

AP Chemistry Curriculum Framework

  • Big Idea 1: The chemical elements are fundamental building materials of matter, and all matter can be understood in terms of arrangements of atoms. These atoms retain their identity in chemical reactions.
    • 1.1 The student can justify the observation that the ratio of the masses of the constituent elements in any pure sample of that compound is always identical on the basis of the atomic molecular theory.
    • 1.2 The student is able to select and apply mathematical routines to mass data to identify or infer the composition of pure substances and/or mixtures.
    • 1.3 The student is able to select and apply mathematical relationships to mass data in order to justify a claim regarding the identity and/or estimated purity of a substance.
    • 1.4 The student is able to connect the number of particles, moles, mass, and volume of substances to one another, both qualitatively and quantitatively.
    • 1.18 The student is able to apply conservation of atoms to the rearrangement of atoms in various processes.
  • Big Idea 2: Chemical and physical properties of materials can be explained by the structure and the arrangement of atoms, ions, or molecules and the forces between them.
    • 2.6 The student can apply mathematical relationships or estimation to determine macroscopic variables for ideal gases.
  • Big Idea 3: Changes in matter involve the rearrangement and/or reorganization of atoms and/or the transfer of electrons.
    • 3.1 Students can translate among macroscopic observations of change, chemical equations, and particle views.
    • 3.2 The student can translate an observed chemical change into a balanced chemical equation and justify the choice of equation type (molecular, ionic, or net ionic) in terms of utility for the given circumstances.
    • 3.3 The student is able to use stoichiometric calculations to predict the results of performing a reaction in the laboratory and/or to analyze deviations from the expected results.

Objectives

By the end of this lesson, students should be able to

  • write a net ionic equation.
  • determine the molar mass of a reactant from the mass of its product.

Chemistry Topics

This lesson supports students’ understanding of

  • Stoichiometry
  • Identifying an unknown
  • Net ionic equations
  • Atomic mass
  • Percent composition

Time

Teacher Preparation: 20–30 minutes

Lesson: 40–60 minutes

Materials

  • Sodium carbonate
  • Sodium bicarbonate
  • Calcium carbonate (about 5 g each)
  • 2-M HCl
  • Test tubes (3)
  • Stirring rod
  • Erlenmeyer flask (125 ml)
  • Graduated cylinder (50 mL or 100 mL)
  • 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.
  • 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

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

  • Sodium carbonate
  • Sodium bicarbonate
  • Calcium carbonate
  • 2-M HCl
  • Test tubes (3)
  • Stirring rod
  • Erlenmeyer flask (125-mL)
  • Graduated cylinder (50-mL or 100-mL)
  • 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 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. Write the net ionic equations for each reaction of the carbonate compound plus the acid.
  4. 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.
  5. Calculate the percentage of carbon in each of the three compounds.
  6. 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.
  7. Why is it only necessary to wash out your flask, but not to completely dry it before testing the second and third compounds?