Materials Included In Kit

Additional Materials Required

Prelab Preparation

Copper(II) Sulfate Pentahydrate Stock Solution for Part II

1. Mass 25.00 g of copper(II) sulfate pentahydrate on a balance in a clean, dry weighing dish.
2. Transfer the solid into a clean, dry 500-mL volumetric flask using a funnel. Use a wash bottle with distilled or deionized (DI) water to rinse any remaining solid from the weighing dish into the flask.
3. Slowly add more DI water to the volumetric flask until the flask is two-thirds full. Place the cap on the volumetric flask and invert several times to mix.
4. Continue filling the volumetric flask until the liquid level is almost to the 500-mL mark. Fill to the mark with a pipet so no liquid splashes up on the sides of the flask. Fill until the bottom of the meniscus is exactly at the 500.0-mL mark.
5. Repeat steps 1–4 as needed depending on class size.
6. Transfer solution to suitable container(s) for classroom distribution.

Unknown Basic Copper Carbonate Solution

7. In a 500-mL erlenmeyer flask, dilute 0.5 M H₂SO₄ to 0.2 M H₂SO₄ (120 mL of 0.5 M H₂SO₄ will make 300 mL of 0.2 M H₂SO₄). Remember: when diluting acids, always add acid to water.
8. Mass 3.25 g of basic copper carbonate on a balance in a clean, dry weighing dish.
9. Transfer the solid to a clean, dry 250-mL volumetric flask using a fume hood. Use a pipet with 0.2 M sulfuric acid to rinse any remaining solid from the weighing dish into the flask.
10. Slowly add more 0.2 M sulfuric acid to the volumetric flask until the flask is two-thirds full. Place the cap on the volumetric flask and invert it several times to mix.
11. Continue filling the volumetric flask until the liquid level is almost to the 250-mL mark. Fill to the mark with a pipet so no liquid splashes up on the sides of the flask. Fill until the bottom of the meniscus is exactly at the 250.0-mL mark.
12. Transfer solution to suitable container(s) for classroom distribution.

Safety Precautions

Hydrochloric and sulfuric acid solutions are toxic and corrosive to eyes and skin tissue. Copper carbonate is slightly toxic by ingestion and inhalation. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Many states regulate or limit the amount of copper(II) salts that may be disposed of down the drain with excess water, see Lab Tips section. One suggested method is solutions from Part I may be neutralized and disposed of according to the Flinn Suggested Disposal Method #24a.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. Both parts of this laboratory activity can reasonably be completed in one 50-minute class period. The prelaboratory assignment may be completed before coming to lab, and the data compilation and calculations may be completed the day after the lab.
• Stock solution should be prepared ahead of time by instructor to produce more accurate results.
• Basic copper carbonate has two different forms. CuCO₃•Cu(OH)₂ [also written Cu₂(OH)₂CO₃] is found in nature in the form of the mineral malachite and is the more common of the two basic copper carbonate forms. It is a dark green crystal, m.p. of 2000 °C, insoluble in cold water, and decomposes in hot water. 2CuCO₃•Cu(OH)₂ [also written Cu₃(OH)₂(CO₃)₂] is found in nature in the form of the mineral azurite. This less common form is a blue crystal with a melting point of about 2200 °C with similar solubility properties as the other basic copper carbonate form.
• In Part II, proper analytical lab technique would require calibrated pipets and volumetric flasks. The availability of a classroom set may be prohibitive. Graduated cylinders are sufficient for the precision reported in this experiment.
• Many states regulate or limit the amount of copper(II) salts that may be disposed of down the drain with excess water. Always check with your local or state agencies before disposing of any substances down the drain. Copper(II) ions may be precipitated as insoluble copper phosphate for disposal method #26a, solid waste in a landfill. Please call or write Flinn Scientific to request a free copy of Precipitation and Disposal of Copper(II) Solutions, Publication No. 10870.

Teacher Tips

• Many empirical formulas are easy for students to predict after learning about valence electrons, this empirical formula for copper carbonate is not obvious and encourages students to analyze experimental data leading to a decision.
• If students know the dilution equation, have them calculate the molarity for each of the standard solutions prepared in Part II by removing the answers on the data table. Alternatively, review with students how the concentrations were calculated.
• If colorimeters or spectrophotometers are available for Part II, a Beer’s Law calibration curve of absorbance versus concentration can be constructed. The percent copper in the basic copper carbonate can be determined using the calibration curve.
• “The empirical formula gives the ratio of atoms in a compound and does not necessarily represent the actual number of atoms in a molecule or formula unit.” If a student asks, there is a “teachable moment” ready and waiting to introduce the difference between empirical formulas and molecular formulas. Molecular formulas tell us the actual number of atoms or formulas in a single molecule of a compound. In order to find the molecular formula of a compound whose empirical formula is known, the molar or molecular mass of the compound must also be known.
• “Identifying an Unknown Metal Carbonate—A Stoichiometry Inquiry Lab,” Flinn Scientific Catalog No. AP7247, uses a two-part experiment to determine which group I metal carbonate was present in the original starting sample. After completing the challenge of determining the empirical of copper carbonate this is the perfect inquiry challenge lab to determine unknowns while developing laboratory technique in titration and problem-solving skills.
• “A Colorful Decomposition of a Carbonate,” Flinn Scientific Catalog No. AP6602, is a chemical demonstration kit which challenges students to determine the chemical reaction that leads to dramatic color changes upon heating copper carbonate.

Answers to Prelab Questions

1. Write the empirical formula for the following compounds and calculate the percent composition of copper:
   1. Copper(I) chloride

      CuCl
      Cu/CuCl x 100% = 63.55/(63.55 + 35.45) x 100% = 64.19% Cu

   2. Copper(II) chloride

      CuCl₂
      Cu/CuCl₂ x 100 = 63.55/[63.55 + (2 x 35.45)] x 100% = 47.27% Cu

2. Calculate the molar mass, percent composition of copper and percent of carbon dioxide from carbonate ion in these compounds:
   {12166_Answers_Figure_1}
3. An unknown metal carbonate was used in the Part I Procedure. Using the data reported below calculate:
   1. The mass of released carbon dioxide
   2. The percent CO₂ from carbonate ion in the sample
      {12166_Answers_Table_4}

      Mass of basic copper carbonate analyzed = 53.127 g – 51.154 g = 1.973 g
      Mass of HCl added to beaker = 31.876 g – 11.812 g = 20.064 g
      Mass of beaker + basic copper carbonate sample + HCl = 51.154 g + 1.973 g + 20.064 g = 73.191 g
      Mass of released CO2 = 73.191 g – 71.985 g = 1.206 g
      Percent CO₂ in basic copper carbonate sample = 1.206 g/1.973 g x 100% = 61.13%

Sample Data

Part I. Gas Evolution Method

Part II. Colorimetric Comparison

Answers to Questions

1. Show the calculations for the gas evolution method data table.
   1. Mass of basic copper carbonate analyzed 69.82 g – 68.65 g = 1.17 g
   2. Mass of HCl added to beaker 38.03 g – 27.50 g = 10.53 g
   3. Mass of beaker + basic copper carbonate sample + HCl 68.65 g + 1.17 g + 10.53 g = 80.35 g
   4. Mass of released CO₂ 80.35 g – 80.12 g = 0.23 g
   5. Percent CO₂ in basic copper carbonate sample 0.23 g /1.17 g x 100% = 19.7%
2. Calculate the percent copper based on the results from Part II, the colorimetric comparison. Example: if the color of the unknown basic copper carbonate solution most closely resembled test tube 2, which has a concentration of 0.16 M, the calculation would be:
   {12166_Answers_Equation_2}
3. Fill in the following table and identify the most likely form of basic copper carbonate that was tested in this lab. Write a short paragraph explaining your choice and describe the supporting evidence quantitatively.
   {12166_Answers_Table_5}

   The %CO₂ and the %Cu are both closer to the theoretical values for Cu₂(OH)₂CO₃, leading to the conclusion that the analyzed basic copper carbonate is indeed Cu₂(OH)₂CO₃. Students may not have a clear answer as to which basic copper carbonate sample was analyzed but it is important that their answer matches their data and shows their reasoning process.

References

Sheeran, Daniel; Copper Content in Synthetic Copper Carbonate: A Statistical Comparison of Experimental and Expected Results; Eastern Illinois University: Charleston, IL; J. Chem. Ed. 1998; Vol. 75 No. 4, pp 453–456.

Introduction

The empirical formulas of many ionic compounds are obvious if the charges on the ions are known. This is not the case with “copper carbonate,” however. Its empirical formula can be determined scientifically by investigating the parts of the compound to the composition of the whole.

Concepts

• Empirical formula
• Percent composition
• Standard dilution
• Colorimetric comparison

Background

The composition of a chemical compound—what it is made of—can be described in several different ways. The percent composition gives the percent by mass of each element in the compound and is the simplest way experimentally to describe the composition of a substance. The elements in a given compound are always present in the same proportion by mass, regardless of the source of the compound or how it is prepared. Calcium carbonate, for example, contains calcium, carbon and oxygen. It is present in eggshells and seashells, chalk and limestone, minerals and pearls. Regardless of where the calcium carbonate originates the mass percentage of the three elements is always the same: 40% calcium, 12% carbon and 48% oxygen.

The percent composition of a compound tells us what elements are present in the compound and their mass ratio. The empirical formula of the compound lists the elements in the compound and the simplest whole number ratio in which the compound can form. For example, the empirical formula of calcium carbonate is CaCO₃—one part calcium, one part carbon and three parts oxygen. The empirical formula gives the ratio of atoms in a compound and does not necessarily represent the actual number of atoms in a molecule or formula unit.

In this lab activity, percent by weight of copper and carbon dioxide in basic copper carbonate will be determined experimentally to identify the composition and empirical formula of this compound, which is derived from a mineral. There are two different compounds that are described as copper carbonate. One compound, CuCO₃•Cu(OH)₂, also written as Cu₂(OH)₂CO₃, is found in nature in the form of the mineral malachite. The second form, 2CuCO₃•Cu(OH)₂, also written as Cu₃(OH)₂(CO₃)₂, is found in nature in the form of the mineral azurite.

Part I. Gas Evolution Method
Two moles of hydrochloric acid will completely react with one mole of carbonate ion, producing one mole of carbon dioxide gas. If the reaction takes place in a beaker or flask, the carbon dioxide will be lost to the atmosphere and the total mass of the flask and its contents will decrease by the mass of the carbon dioxide that is released (Equation 1). During data analysis the theoretical percent carbon dioxide for each of two possible formulas of basic copper carbonate will be compared with the experimental results to identify the composition of the compound.

Part II. Colorimetric Comparison
In Part II of the experiment a series of diluted copper solutions having known concentrations are prepared and the percent copper in a solution of the unknown copper carbonate can be analyzed colorimetrically. The amount of solute that is dissolved in a given quantity of solvent is called the concentration of the solution. A dilute solution contains only a small amount of solute in a given amount of solution, while a concentrated solution contains a large amount of solute in a given amount of solution. An important problem chemists encounter in the lab is how to determine the concentration of an unknown solution. If the solution is colored by a solute, the concentration of an unknown solution can be determined by measuring the intensity of the color. This can be done by visual inspection through color comparison using a series of samples of known concentration that are both lighter and darker than the unknown sample. To increase accuracy in comparing color intensity, the solutions should have a constant total volume and all the samples should be placed in the same type of container. If a more quantitative approach is desired, a special instrument called a colorimeter or spectrophotometer can be used to measure the absorbance of visible light that gives the solution its color. When preparing the samples of known concentration it is important to be as accurate as possible. In industry, chemists use calibrated glassware such as pipets and volumetric flasks. If volumetric flasks are not available, graduated cylinders may also be used with proper laboratory technique.

Experiment Overview

There are two different formulas for copper carbonate: basic, as it is found in nature or prepared in the lab. To determine the correct formula, a two-part experiment is conducted. In Part I of the experiment, the percent carbon dioxide is calculated based on weight loss after reaction with acid. In Part II, a colorimetric calibration is done by comparing the amount of copper in the unknown with a series of standard solutions.

Materials

Prelab Questions

1. Write the empirical formula for the following compounds and calculate the percent composition of copper:
   1. Copper(I) chloride
   2. Copper(II) chloride
2. Calculate the molar mass, percent composition of copper and percent of carbon dioxide from carbonate ion in these compounds:
   1. Cu₂(OH)₂CO₃
   2. Cu₃(OH)₂(CO₃)₂
3. An unknown basic copper carbonate was used in the Part I Procedure. Using the data reported calculate:
   1. The mass of released carbon dioxide
   2. The percent CO₂ from carbonate ion in the sample
      {12166_PreLabQuestions_Table_1}

Safety Precautions

Hydrochloric acid solution is toxic and corrosive to eyes and skin tissue. Copper carbonate is slightly toxic by ingestion and inhalation. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.

Procedure

Part I. Gas Evolution Method

1. Mass a 125-mL Erlenmeyer flask on a balance and record the value in the data table.
2. In a tared weighing dish, obtain approximately 1.5 g of the copper carbonate sample. Measure and record the precise mass of the metal carbonate in the data table.
3. Transfer the copper carbonate sample to the 125-mL Erlenmeyer flask.
4. Use a 25-mL graduated cylinder to obtain 15 mL of 2 M hydrochloric acid solution. Measure the combined mass of the cylinder and the acid solution on a balance and record the combined mass in the data table.
5. Slowly pour the hydrochloric acid into the Erlenmeyer flask. Allow the reaction of the HCl and the copper carbonate to go to completion. Measure and record the mass of the empty cylinder after the HCl has been added to the flask.
6. Once the reaction appears complete, swirl the flask to allow any further reaction to take place.
7. Measure the mass of the Erlenmeyer flask and its contents and record the mass in the data table.
8. Repeat the procedure if instructed by the teacher.

Part II. Colorimetric Comparison

9. Place seven clean and dry test tubes in a test tube rack and label them 1–6 and U.
10. Label one pipet CuSO₄ and use it to transfer the stock solution only.
11. Label another pipet H₂O and use it to transfer DI water only.
12. Finally, label a third pipet U and use this to transfer the unknown basic copper carbonate solution only.
13. Using a 10-mL graduated cylinder, measure and pour 10.0 mL of the basic copper carbonate solution into test tube U. Notice in the data table the mass of sample and volume of solution that were used to prepare the unknown solution.
14. Using a 10-mL graduated cylinder, measure and pour 10.0 mL of the 0.20 M copper(II) sulfate stock solution into test tube 1.
15. Rinse the graduated cylinder with water and dry it with a paper towel.
16. Using the Beral-type pipet labeled CuSO4, fill the 10 mL-graduated cylinder exactly to the 8.0 mL mark with the 0.20 M copper(II) sulfate stock solution. Try not to get any drops of solution on the sides of the cylinder. Make sure that the bottom of the meniscus sits exactly at the 8.0-mL mark.
17. Use the pipet labeled H₂O to carefully fill the graduated cylinder to exactly the 10.0-mL mark with DI water. Do not overfill.
18. Transfer the solution to test tube 2.
19. Repeat steps 15–17 using 7.0 mL of stock solution and transfer the diluted solution to test tube 3.
20. Repeat steps 15–17 using 6.0 mL of stock solution and transfer the diluted solution to test tube 4.
21. Repeat steps 15–17 using 4.0 mL of stock solution and transfer the diluted solution to test tube 5.
22. Repeat steps 15–17 using 2.0 mL of stock solution and transfer the diluted solution to test tube 6.
23. Compare the U sample (unknown basic copper carbonate solution) to the six known samples. Choose the known sample 1–6 whose color intensity most closely matches the unknown solution. Place a piece of white paper under, and another on the sides of the test tubes if needed, to block any color interference. When comparing the color of the solutions, look straight down into the test tubes.
24. If selecting the best color match is difficult or if desired, obtain a clean reaction plate and place it on a white sheet of paper. Transfer the same volume of each of the five known sample solutions and of the unknown to the reaction plate. Look straight down into the reaction plate to determine which sample best matches the unknown. Record the color comparison choice in the data table.

Student Worksheet PDF

12166_Student1.pdf