Materials Included In Kit

Additional Materials Required

Safety Precautions

Hydrochloric acid solution is toxic and corrosive to eyes and skin tissue. Lithium carbonate is corrosive to eyes and the respiratory tract; moderately toxic by ingestion and a possible teratogen. Potassium carbonate is a body tissue irritant. 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 consult 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. Waste solutions from Part 1 may be neutralized and disposed of according to Flinn Suggested disposal Method #24b. The titrated solutions may be disposed of according the Flinn Suggested Disposal Method #26b.

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 two 50-minute class periods. The prelaboratory assignment should be completed before coming to lab, and the data compilation and calculations can be completed the day after the lab.
• In Part 1, for a 2.00 g sample, the percent deviation in the molar mass determination ranges from 1% to 10%.
• In Part 1, the first analysis based on weight loss generally leads to too large a molar mass value by 5 to 15%. The major error is too small a mass loss, presumably due to insufficient swirling of the flask containing the reaction mixture.
• In Part 2, the metal carbonate solutions can be made up before class. Students can then use 50-mL aliquots of their assigned unknown solution.
• The titration lab teaches students how to use volumetric glassware and encourages them to develop good laboratory technique. The titration can be modified to have students also collect pH data using a pH probe. The plot of this pH data will show the students the shape of the titration curve of this weak base titrated by a strong acid. This curve shows the two-step neutralization happening in this titration. The first neutralization occurs as the hydrochloric acid converts the carbonate ion, CO₃^2–, to the hydrogen carbonate, or bicarbonate ion, HCO₃^–:

  CO₃^2–(aq) + HCl(aq) → HCO₃^–(aq) + Cl^–(aq)

  This occurs at a pH value around 7 (see A, Figure 2).
  The subsequent HCO₃^– reacts with a second mole of HCl to produce carbon dioxide and water;

  HCO₃^–(aq) + HCl(aq) → CO₂(aq) + H₂O(l) + Cl^–(aq)

  The endpoint occurs at a pH value of about 4 (see B, Figure 2), which is in the transition range for bromcresol green (pH = 3.8–5.4).
  {12678_Hints_Figure_2}
• In Part 2, the solution is heated just prior to the end of the titration to expel any dissolved carbon dioxide.
• For greater accuracy, the 0.10 M HCl solution supplied with the kit may be standardized using a primary standard. Reagent grade sodium carbonate is considered a primary standard for bases. Dry about 2.0 g of the pure sodium carbonate in an oven at 110 °C for one hour. Weight out a 0.25 g portion of the sodium carbonate and dissolve it in about 100 mL of distilled or deionized water. Titrate this solution with the HCl solution to determine the HCl solution molarity.

Teacher Tips

• The level of student independence in this guided-inquiry lab can be modified by requiring the students to develop a procedure for Part 1, instead of giving them the directions. Let them decide on their own how to calculate the molar mass of the metal carbonate from the mass loss.
• Quantitative analysis represents a nearly invisible application of chemistry in our daily lives. To illustrate the hidden importance of quantitative analysis, ask students how they would feel if they could not trust that the medicines they take had been tested to assure their quality and safety.

Answers to Prelab Questions

An unknown metal carbonate was analyzed by the gas evolution method and yielded the following data.

1. The equation for the reaction of a Group 1 metal carbonate with hydrochloric acid is:
   {12678_PreLab_Equation_5}
   1. From the mass of CO₂ released, calculate the moles of CO₂ produced in the reaction.

      Moles of CO₂ produced = mass of released CO₂/molar mass of CO₂

      = 1.206 g/44.010 g/mole
      = 0.0274 mole CO₂

   2. Calculate the molar mass of the unknown metal carbonate, M₂CO₃.

      Molar mass of M₂CO₃ = mass of M₂CO₃/moles of CO₂ released

      = 1.973 g/0.274 mole
      = 72.0 g/mole

   3. What is the identity of the unknown metal carbonate?

      The molar mass of the first 3 group 1 metal carbonates are

      Li₂CO₃ = 73.89 g/mol
      Na₂CO₃ = 105.99 g/mol
      K₂CO₃ = 138.21 g/mol

      The unknown carbonate is probably lithium carbonate.

2. In Part 2 of the laboratory, the group 1 metal carbonate is dissolved in water and the carbonate ion is titrated with a 0.1 M hydrochloric acid solution. The overall acid—base reaction is
   {12678_PreLab_Equation_6}
   At the endpoint, the number of moles of HCl added will be equal to twice the moles of dissolved metal carbonate. Since the number of moles of HCl is equal to the molarity of the acid times the volume of acid titrated,
   {12678_PreLab_Equation_7}
   calculate the mass of Li₂CO₃ that would be neutralized by 40 mL of 0.1 M HCl.

   Moles(acid) = (0.0400 L(HCl) x 0.10 mole/L = 0.0040 mole (acid)
   Moles (Li₂CO₃) = ½ mole (HCl) = 0.0020 mole
   Mass (Li₂CO₃) = 0.0020 mole x 73.89 g/mole = 0.15 g of (Li₂CO₃)

Sample Data

Gas Evolution Reaction

Titration Analysis

Mass of solid M₂CO₃ (Li₂CO₃) ___1.05___g Mass of solid M₂CO₃ (Na₂CO₃) ___1.03___g

Answers to Questions

1. Using the data obtained in Part 1, calculate the number of moles of carbon dioxide, CO₂, produced in the reaction.

   Trial 1: 1.21 g/44 g/mole = 2.75 x 10^–2 moles CO₂
   Trial 2: 0.68 g/44 g/mole = 1.55 x 10^–2 moles CO₂

2. Calculate the molar mass of the unknown Group 1 metal carbonate and identify the metal.

   Molar mass of M₂CO₃ = mass of M₂CO₃/moles of CO₂ released
   
   Trial 1: 2.00 g/2.75 x 10^–2 moles CO₂ = 72.7 g/mole
   The unknown carbonate is lithium carbonate.
   
   Trial 2: 2.02 g/1.55 x 10^–2 moles CO₂ = 130 g/mole
   The unknown carbonate is potassium carbonate.

3. Calculate the percent error in the experimental determination of the molar mass.
   {12678_Answers_Equation_8}
4. Using the data obtained in Part 2, calculate the moles of hydrochloric acid used to neutralize the unknown Group 1 metal carbonate dissolved in the 50 mL sample for each trial titration.

   For Li₂CO₃ data:

   Trial 1: 0.02820 liters x 0.10 moles/ liter = 2.820 x 10^–3 moles HCl
   Trial 2: 0.02910 liters x 0.10 moles/ liter = 2.910 x 10^–3 moles HCl

   For Na₂CO₃ data:

   Trial 1: 0.01815 liters x 0.10 moles/ liter = 1.815 x 10^–3 moles HCl
   Trial 2: 0.01780 liters x 0.10 moles/ liter = 1.780 x 10^–3 moles HCl

5. For each trial, calculate the total moles of the unknown group 1 metal carbonate originally dissolved in the 500 mL of distilled or deionized water.

   For Li₂CO₃ data:

   Trial 1: (2.820 x 10^–3 moles HCl) x (1 mole Li₂CO₃/2 moles HCl) = 1.42 x 10^–3 mole Li₂CO₃
   (1.42 x 10^–3 mole Li₂CO₃/50 mL) x 500 mL = 1.42 x 10^–2 moles Li₂CO₃
   
   Trial 2: (2.910 x 10^–3 moles HCl) x (1 mole Li₂CO₃/2 moles HCl) = 1.46 x 10^–3 mole Li₂CO₃
   (1.46 x 10^–3 mole Li₂CO₃/50 mL) x 500 mL = 1.46 x 10^–2 moles Li₂CO₃

   For Na₂CO₃ data:

   Trial 1: (1.815 x 10^–3 moles HCl) x (1 mole Na₂CO₃/2 moles HCl) = 9.08 x 10^–2 mole Na₂CO₃
   (9.08 x 10^–2 mole Na₂CO₃/50 mL) x 500 mL = 9.08 x 10^–1 moles Na₂CO₃
   
   Trial 2: (1.780 x 10^–3 moles HCl) x (1 mole Na₂CO₃/2 moles HCl) = 8.90 x 10^–4 mole Na₂CO₃
   (8.90 x 10^–2 mole Na₂CO₃/50 mL) x 500 mL = 8.90 x 10^–3 moles Na₂CO₃

6. Calculate the molar mass of the unknown Group 1 metal carbonate and identify the metal.

   For Li₂CO₃ data:

   Trial 1: 1.05 g Li₂CO₃/1.42 x 10^–2 moles Li₂CO₃ = 73.9 g/mole
   Trial 2: 1.05 g Li₂CO₃/1.46 x 10^–2 moles Li₂CO₃ = 71.9 g/mole

   For Na₂CO₃ data:

   Trial 1: 1.03 g Na₂CO₃/9.08 x 10^–3 moles Na₂CO₃ = 113 g/mole
   Trial 2: 1.03 g Na₂CO₃/8.90 x 10^–3 moles Na₂CO₃ = 116 g/mole

7. Calculate the percent error in the experimental determination of the molar mass.

   For Li₂CO₃ data:

   {12678_Answers_Equation_10}
   For Na₂CO₃ data:
   {12678_Answers_Equation_11}

References

Special thanks to Emily Dudek, Professor Emerita, Brandeis University, for providing the idea and the instructions for this activity.

Introduction

How do chemists determine the identity of a compound? A variety of analytical techniques and procedures, ranging from instrumental methods such as spectroscopy and chromatography, to more classical processes, such as qualitative and gravimetric analysis, have been created to accomplish that task. In this laboratory, the identity of a Group 1 metal carbonate will be determined using two complementary methods—change in mass due to loss of CO₂ and acid–base titration.

Concepts

• Stoichiometry
• Acid–base titrations
• Ideal gas law
• Limiting reactant

Background

The general formula for a Group 1 metal carbonate is M₂CO₃. The members of this family, including Li₂CO₃, Na₂CO₃, K₂CO₃, Rb₂CO₃ and Cs₂CO₃, are all white, crystalline powders. All of the compounds are water soluble, at least to some extent.

Part 1. Gas Evolution Method
In solution, the Group 1 metal carbonates will be reacted with hydrochloric acid to product a salt, (MCl), carbon dioxide gas, and water (see Equation 1).

Two moles of acid will completely react with one mole of metal carbonate, 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.

Part 2. Titration
The carbonate ion, CO₃^2–, acts as a weak base in solution. It can be titrated with a strong acid such as HCl according to Equations 2–4. Titration is a method of volumetric analysis—the use of volume measurements to analyze an unknown. Acid–base titration is used to analyze the amount of acid or base in a sample or solution. An unknown metal carbonate is “titrated” by slowly adding, dropwise, a standard solution of hydrochloric acid to a solution containing a preweighed amount of the metal carbonate. (A standard solution is one whose concentration is accurately known.) The titrant, hydrochloric acid in this case, reacts with and consumes the carbonate ion via a neutralization reaction (Equation 4). The exact volume of acid needed to react completely with the basic metal carbonate is measured. This is called the equivalence point of the titration—the point at which stoichiometric amounts of the acid and base have reacted.

Knowing the exact concentration and volume of added titrant gives the number of moles of hydrochloric acid. The latter, in turn, is related by stoichiometry to the number of moles of carbonate ion initially present in the unknown.

Indicators are usually added to acid–base titrations to detect the equivalence point. The endpoint of the titration is the point at which the indicator changes color and signals that the equivalence point has indeed been reached. In the case of the neutralization reaction shown in Equation 4, the pH of the solution would be basic (>7) before the equivalence point and acidic (<7) after the equivalence point. bromcresol green, which changes from blue to green in the ph range 5.4–3.8, is used as the indicator for titrations involving carbonate ions. the solution is briefly heated just prior to the endpoint to expel any dissolved carbon dioxide.>

Materials

Prelab Questions

An unknown metal carbonate was analyzed by the gas evolution method and yielded the following data.

1. The equation for the reaction of a Group 1 metal carbonate with hydrochloric acid is:
   {12678_PreLab_Equation_5}
   1. From the mass of CO₂ released, calculate the moles of CO₂ produced in the reaction.
   2. Calculate the molar mass of the unknown metal carbonate, M₂CO₃.
   3. What is the identity of the unknown metal carbonate?
2. In Part 2 of the laboratory, the group 1 metal carbonate is dissolved in water and the carbonate ion is titrated with a 0.1 M hydrochloric acid solution. The overall acid–base reaction is
   {12678_PreLab_Equation_6}
   At the endpoint, the number of moles of HCl added will be equal to twice the moles of dissolved metal carbonate. Since the number of moles of HCl is equal to the molarity of the acid times the volume of acid titrated,
   {12678_PreLab_Equation_7}
   calculate the mass of Li₂CO₃ that would be neutralized by 40 mL of 0.1 M HCl.

Safety Precautions

Hydrochloric acid solution is toxic and corrosive to eyes and skin tissue. Lithium carbonate is corrosive to eyes and the respiratory tract; it is moderately toxic by ingestion and is a possible teratogen. Potassium carbonate is a body tissue irritant. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Part 1. Gas Evolution Reaction

1. Measure the mass of a 125-mL Erlenmeyer flask on a balance and record the value in the data table.
2. In a tared weighing dish, mass approximately 2 g of the unknown Group 1 metal carbonate sample, M₂CO₃, on a balance. Record the precise mass of the metal carbonate in the data table.
3. Transfer the metal carbonate sample to the 125-mL Erlenmeyer flask.
4. Use a 25-mL graduated cylinder to obtain 20 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 M₂CO₃ 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 to by the teacher.

Part 2. Titration

1. Obtain approximately 1 g of the unknown Group 1 metal carbonate sample, M₂CO₃, in a tared weighing dish. Measure and record the precise mass of the metal carbonate sample in the data table.
2. Transfer the metal carbonate sample to a 500-mL Erlenmeyer flask. Using a 500-mL graduated cylinder, add 500 mL of distilled or deionized water and a stir bar to the same flask, and mix the solution on a magnetic stirrer.
3. Obtain a clean 50-mL buret and rinse it with two 5-mL portions of 0.10 M HCl solution.
4. Use the buret clamp to secure the buret to the ring stand and place the 50-mL beaker under the buret tip. Fill the buret to above the zero mark with the 0.10 M HCl solution. Open the stopcock to allow any air bubbles to escape from the tip. Close the stopcock when the liquid level in the buret is between the 0- and 5-mL mark.
5. Obtain approximately 50 mL of distilled or deionized water in a 125-mL Erlenmeyer flask and add 5 drops of bromcresol green indicator.
6. Record the precise level (initial volume) of the 0.10 M HCl solution in the buret in the data table. Note: Volumes are read from the top down in a buret. Always read from the bottom of the meniscus and remember to include the appropriate number of significant figures (see Figure 1).
   {12678_Procedure_Figure_1}
7. Place the flask containing distilled or deionized water under the buret so that the tip of the buret is inside the mouth of the flask. Place a piece of white paper under the flask to make it easier to detect the color change of the indicator at the endpoint.
8. Carefully add the HCl solution one drop at a time to the flask. Swirl the flask after each drop is added. Stop when the solution color just changes from blue-green to yellow-green. Record the buret volume in the Data Table. Save this “control” flask as a color standard for detecting the endpoint in step 12.
9. Using a 50-mL graduated cylinder, transfer 50.0 mL of the metal carbonate solution to a clean 125-mL Erlenmeyer flask. Record this volume in the data table. Add 5 drops of bromcresol green indicator solution to the flask.
10. Record the initial volume of the HCl solution in the buret. Titrate the carbonate solution with the HCl solution to an intermediate green or slightly aqua color.
11. Stop the titration and place the flask on a hot plate. Heat the solution to a gently boil for two to three minutes. Use beaker tongs or heat-resistant gloves to remove the flask from the hot plate. Place the flask in an ice-bath or under cold running water to cool the solution.
12. When the solution has cooled back to room temperature, complete the titration of the carbonate solution to match the yellow-green endpoint obtained in step 8. Record the final volume of the HCl solution in the buret in the data table.
13. Repeat the procedure with a new 50-mL sample of the metal carbonate solution if instructed to do so by the teacher.
14. Dispose of the waste solutions as instructed by the teacher.

Student Worksheet PDF

12678_Student1.pdf