Materials Included In Kit

Additional Materials Required

Safety Precautions

Potassium carbonate is slightly toxic by ingestion and is a skin irritant. Calcium chloride is slightly toxic by ingestion. Handle the crucible only with tongs. Do not touch the crucible with fingers or hands. There is a significant burn hazard associated with handling a crucible—remember that a hot crucible looks like a cold one. Wear chemical splash goggles and chemical-resistant gloves and 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 regulation that may apply, before proceeding. The sodium carbonate, potassium carbonate, calcium chloride solid, and the calcium carbonate may all be disposed of according to Flinn Suggested Disposal Method #26a. The calcium chloride solutions may be disposed of according to Flinn Suggested Disposal Method #26b.

Lab Hints

• The actual laboratory work for this experiment takes only about 50 minutes, but most students will require additional time. If more than one lab period is needed, a convenient stopping point is step 23. The watch glasses and precipitates may be left in the drying oven overnight.
• Two of the most basic and important techniques to master in chemical analysis are filtering and decanting. These two techniques of quantitative transfer occur as crucial steps in many analytical determinations. To make sure the filtering speed is as rapid as possible, the filter paper must be seated properly in the funnel. The filter paper should make complete contact with the sides of the funnel and drain with few, if any, air bubbles in the stem. This is done by folding the filter paper as shown in Figure 6 and tearing off the corner. This allows the subsequent cone of filter paper to be placed smoothly against the sides of the funnel. Place the filter paper cone in the funnel. Wet the filter paper thoroughly with distilled or deionized water and use your fingers to smooth the paper against the sides of the funnel until no air bubbles are visible in the stem.
  {13873_Hints_Figure_6}
  Proper decanting technique ensures that the precipitate is transferred from the beaker to the filter paper and little, if any, is lost during the transfer. Start by holding a stir rod against the lip of the beaker and pour the liquid from the beaker into the funnel. The liquid should run down the rod and into the funnel without splashing (Figure 7). Keep the level of the liquid in the funnel below the top of the filter paper.
  {13873_Hints_Figure_7}
  When all the liquid has been transferred to the funnel, begin transferring the remaining precipitate from the beaker to the funnel. Add a stream of distilled or deionized water to the beaker. Use a rubber policeman on the end of the stir rod to loosen any precipitate clinging to the beaker. Rinse the rubber policeman with a stream of distilled or deionized water, swirl the beaker to suspend the precipitate, and transfer the suspension to the funnel using the technique outlined in Figure 7. Repeat this rinsing until nearly all the precipitate has been transferred. Rinse the beaker again and transfer the liquid to the flask. Hold the beaker and stir rod as shown in Figure 8 and rinse the sides and bottom of the flask with distilled or deionized water. Rinse at a rate that allows the liquid to flow down the stir rod into the funnel without splashing and doesn’t allow the liquid level to rise above the top of the filter paper. Repeat this rinse until no precipitate is visible in the beaker.
  {13873_Hints_Figure_8}
  Teachers should demonstrate these techniques to the students and allow those not yet proficient sufficient time to practice setting up the filter paper in the filter cone and decanting a liquid from a flask.
• If no drying oven is available, an evaporating dish or large porcelain crucible and Bunsen burner can be used. Place the solid and paper in the evaporating dish or crucible and heat gently over a Bunsen burner for a few minutes. Make sure the paper is not exposed to the flame and do not allow the paper to char. In this manner, the moisture can be driven off without destroying either the filter paper or the precipitated calcium carbonate.

Teacher Tips

• A variation on this laboratory calls for the determination of the percent calcium content in over-the-counter, calcium-enriched tablets. The tablets are weighed and dissolved in solution. A solution of sodium carbonate is added to form the calcium carbonate precipitate. The calcium carbonate is then filtered, dried, and weighed. From the mass of calcium carbonate and the mass of the tablet, the percent calcium content of the tablet can be calculated.
• Give each group a different unknown number, then code the number back to the two unknowns.

Further Extensions

AP^® Standards
This lab fulfills the requirements for the College Board recommended AP experiment #16: Analytical Gravimetric Determination. In addition, this lab provides the recommended familiarity with the process of gravimetric analysis.

Answers to Prelab Questions

An unknown metal carbonate was analyzed gravimetrically and yielded the following data.

1. From the mass of CaCO₃, calculate the moles of CaCO₃ precipitated.
   {13873_PreLabAnswers_Equation_4}
2. Calculate the molar mass of the unknown.
   {13873_PreLabAnswers_Equation_5}
3. Calculate the molar mass of the following Group 1 metal carbonates. Formula weight of: a. Li₂CO₃ = 73.89 g/mol b. Na₂CO₃ = 105.99 g/mol c. K₂CO₃ = 138.21 g/mol
4. What is the identity of M₂CO₃? The molar mass of M₂CO₃ was determined to be 107.4 g/mol. Therefore, the unknown carbonate is Na₂CO₃.
5. Calculate the percent error in the molar mass determination of M₂CO₃.
   {13873_PreLabAnswers_Equation_6}

Sample Data

Student data will vary.

Answers to Questions

1. Calculate the moles of precipitated calcium carbonate, CaCO₃. Enter value in data table.
   {13873_Data_Equation_7}
2. Calculate the molar mass of the unknown carbonate. Enter this value in the data table.
   {13873_Data_Equation_8}
3. From the calculated molar mass, identify the unknown. Calculate the percent error in the molar mass value. Enter both values in the data table.

   Formula weight of:

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

   1. Unknown carbonate is K₂CO₃.
      {13873_Data_Equation_9}
   2. Unknown carbonate is Na₂CO₃.
      {13873_Data_Equation_10}

      Percent error = 1.90%

4. Review the procedure and list possible sources of errors that would cause the molar mass of the unknown to be (a) too high (b) too low.
   1. Any steps that would result in the mass of the unknown being too high (i.e., the unknown was not properly dried in steps 4 to 7).

      Any steps that would result in the moles of calcium carbonate being too low (i.e., calcium carbonate solid lost in any of the steps between filtering the precipitate and weighing the dried filter paper and solid).

   2. Any steps that would result in the mass of the unknown being too low (i.e., if the crucible was not dry in step 4, the net mass of the unknown would be too low).

      Any steps that would result in the moles of calcium carbonate being too high (i.e., the filter paper and calcium carbonate were not completely dry in step 28 when measuring the final mass).

Introduction

How do chemists determine the identity of a compound? A large variety of analytical techniques and procedures, ranging from instrumental methods such as spectroscopy and chromatography to more classical processes, such as qualitative and gravimetric analyses, have been created to accomplish that task. In this laboratory, the identity of a Group 1 metal carbonate is determined gravimetrically using a double-replacement precipitation reaction.

Concepts

• Double-replacement reaction
• Gravimetric analysis

Background

In this experiment, an unknown Group 1 metal carbonate, M₂CO₃, is analyzed to determine the identity of the Group 1 metal, M.

A known amount of the soluble unknown carbonate is dissolved in water to dissociate the compound into its ions (Equation 1).

When a solution of calcium chloride, CaCl₂, is added to this metal carbonate solution, a precipitate of calcium carbonate forms (Equation 2). The overall reaction represents a double-replacement reaction with a precipitate formed (Equation 3). The precipitated calcium carbonate is then filtered, dried, and weighed. The moles of calcium carbonate, CaCO₃, are equal to the moles of Group 1 metal carbonate, M₂CO₃, added to the original solution. Dividing the mass of the unknown carbonate by the moles of calcium carbonate yields the formula weight, and thus the identity, of the Group 1 metal carbonate.

Experiment Overview

The purpose of this lab is to determine the identity of a Group 1 metal carbonate compound by gravimetric analysis. The unknown is weighed and dissolved in water. A solution of calcium chloride is added to the metal carbonate solution to precipitate the carbonate ions as calcium carbonate. The precipitate is filtered, dried and weighed. From the data, the formula weight and identity of the unknown metal carbonate is determined.

Materials

Prelab Questions

An unknown metal carbonate was analyzed gravimetrically and yielded the following data.

1. From the mass of CaCO₃, calculate the moles of CaCO₃ precipitated.
2. Calculate the molar mass of the unknown.
3. Calculate the molar mass of the following Group 1 metal carbonates:
   1. Li₂CO₃
   2. Na₂CO₃
   3. K₂CO₃
   4. What is the identity of M₂CO₃?
4. Calculate the percent error in the molar mass determination of M₂CO₃ by comparing the experimentally determined molar mass of M₂CO₃ to the known molar mass of the appropriate metal carbonate.

Safety Precautions

The unknown solids are slightly toxic by ingestion and are skin irritants. Handle the crucible only with tongs. Do not touch the crucible with fingers or hands. There is a significant burn hazard associated with handling a crucible—remember that a hot crucible looks like a cold one. Wear chemical splash goggles and chemical-resistant gloves and apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

1. Obtain a clean, dry 15-mL crucible.
2. Set up a Bunsen burner on a ring stand beneath a ring clamp holding a clay pipe stem triangle. Place the crucible in the clay triangle (see Figure 1). Do NOT light the Bunsen burner.
   {13873_Procedure_Figure_1}
3. Light the Bunsen burner and brush the bottom of the crucible with the burner flame for about one minute. Turn off the Bunsen burner and allow the crucible to cool.
4. Using tongs to handle the crucible, measure the mass of the clean, empty crucible to the nearest 0.001 g. Record the mass in the data table.
5. While the crucible is still on the balance, add approximately 2 g of the unknown carbonate to the crucible. Record the combined mass of the crucible and unknown carbonate in the data table.
6. Place the crucible on the clay triangle as shown in Figure 2. Light the Bunsen burner again and slowly heat the crucible by brushing the bottom of the crucible with the Bunsen burner flame for 2–3 minutes. Set the crucible to cool on a heat-resistant pad.
   {13873_Procedure_Figure_2}
7. Weigh the crucible on an analytical balance. Record the mass in the data table.
8. Repeat steps 6 and 7 until the mass of the crucible and unknown carbonate no longer decreases. Note: The Group 1 metal carbonates are hydroscopic—they absorb water from the air. These heating steps are necessary to ensure the crucible is dry and the carbonate samples are anhydrous when massed.
9. Add the crucible contents to a 400-mL beaker.
10. Add about 200 mL of distilled or deionized water to the beaker and stir to dissolve the unknown carbonate.
11. Add about 125 mL of the 0.2 M CaCl₂ solution to the 400-mL beaker and stir.
12. Let the precipitate settle (5 minutes).
13. Obtain a piece of quantitative filter paper. Weigh the filter paper on the analytical balance. Record the mass of the filter paper in the data table.
14. Fold the filter paper into a cone. First fold the filter paper in half and crease. Next, fold the filter paper almost in half again, leaving about a 5° angle between the folded edges (see Figure 3).
    {13873_Procedure_Figure_3}
15. Tear off the corner of the top edge, open the filter paper into a cone shape, and place the torn corner in the bottom of the cone.
16. Place the cone into the filter funnel. Position the paper tight against the funnel walls and moisten the paper with about 5 mL of deionized water from a wash bottle. Note: After adding the water, use index fingers to seat the filter paper tightly against the sides of the funnel so that little, if any, air gaps are visible in the stem as the water filters through.
17. Set up the ring stand and iron ring and place the funnel in the ring. Let the funnel drain into a second 400-mL beaker (see Figure 4).
    {13873_Procedure_Figure_4}
18. Using a stirring rod, decant the liquid from the 400-mL beaker into the funnel. Be sure to keep the liquid level below the top of the filter paper cone (see Figure 5).
    {13873_Procedure_Figure_5}
19. When all but approximately 10 mL of the liquid has been transferred, swirl the beaker to suspend the precipitated CaCO₃. Transfer this to the funnel, again making sure not to fill the cone above the top of the filter paper.
20. Rinse the flask with small amounts of distilled or deionized water from the wash bottle and then transfer the washings to the filter.
21. When all the solid has been transferred to the filter paper, rinse the solid with three small portions of distilled or deionized water. Allow the funnel to drain completely.
22. Obtain a watch glass. Using a microspatula, take the filter paper out of the funnel and place it in the center of the watch glass. Be careful not to tear the paper or to lose any part of the solid.
23. Using the microspatula, carefully open the filter paper into a circle on the watch glass. Place the watch glass and filter paper in a drying oven set at 110–120 °C.
24. Allow the filter paper to dry for 10–15 minutes. Remove the watch glass from the oven using crucible tongs. Use the spatula to break up the CaCO₃ into small particles.
25. Return the watch glass to the drying oven for an additional 5 minutes.
26. Remove the watch glass from the oven and set it aside to cool.
27. When cool, weigh the filter paper and the solid CaCO₃ on an analytical balance. Record the mass in the data table.
28. Repeat steps 25–27, until the mass readings do not change by more than 0.005 g.

Student Worksheet PDF

13873_Student1.pdf