AP Chemistry Lab Date: _________________________ Chemist: ________________________ Lab Write‐Up Due Date: ___________________ Lab 1: Gravimetric Analysis of a Metal Carbonate BACKGROUND 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. OBJECTIVES In this experiment, you will determine the identity of a Group 1 metal carbonate (M2CO3, where M is the unknown group 1 metal) using gravimetric analysis. MATERIALS 








0.20 M Calcium chloride solution Unknown carbonate sample, M2CO3 Distilled water Analytical balance 400 mL beaker (2) Bunsen burner Crucible, 15 mL Crucible tongs Drying oven









Filter paper Buchner funnel + rubber collar Filter flask + rubber tubing Glass stirring rod 250 mL graduated cylinder Ring stand + ring Spatula Clay triangle Wash bottle Watch glass 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 triangle. Place the crucible in the clay triangle (see Figure 1). 3. Light the Bunsen burner and brush the bottom of the empty crucible with the burner flame for about 1 minute. Turn off the Bunsen burner and allow the crucible to cool. 4. Using crucible tongs to handle the crucible to avoid leaving fingerprints, 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 the unknown carbonate in the Data Table. 6. Place the crucible on the clay triangle. 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. 7. Weigh the crucible on an analytical balance. Record the mass in the Data Table. Lab 1: Gravimetric Analysis of a Metal Carbonate 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, meaning 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 200 mL of distilled water to the beaker and stir to dissolve the unknown carbonate. 11. Add 125 mL of the 0.2 M CaCl2 solution to the 400 mL beaker and stir. 12. Let the precipitate settle (5 min). 13. While the precipitate settles, obtain a piece of filter paper. Weight the filter paper on the analytical balance. Record the mass of the filter paper in the Data Table. 14. Set up a vacuum filtration apparatus. Connect the rubber tubing to the water aspirator (on the sink) directly to a filter flask. Place the Buchner funnel directly into the rubber collar before sealing the filter flask with the rubber collar. 15. Place the filter paper into the Bucher funnel so that it lies completely flat. Turn on the sink attached to your Buchner funnel. The water will run continuously as you filter your liquid. 16. Carefully pour the liquid from the 400 mL beaker into the funnel. When all but approximately 10 mL of the liquid has been transferred, swirl the beaker to suspend the precipitated CaCO3. Transfer this liquid to the funnel. 17. Rinse the flask with small amounts of distilled water from the wash bottle and then transfer the washings to the filter. 18. When all of the solid has been transferred to the filter paper, rinse the solid with three small portions of distilled water. Allow the funnel to drain completely, then turn off the water aspirator. 19. Obtain a watch glass. Using a small spatula, 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. 20. Place the watch glass and the filter paper in a drying oven set at 110‐120 oC. 21. After 15 min of trying, use a spatula to break up the CaCO3 into small pieces. 22. Return the watch glass to the drying oven for an additional 5 minutes. 23. Remove the watch glass from the oven and set it aside to cool. 24. When cool, weigh the filter paper and the solid CaCO3 on an analytical balance. Record the mass in the Data Table. 25. Repeat steps 22‐24 until the mass readings do not change by more than 0.005 g. Adapted from Laboratory Experiments for AP Chemistry by Sally Ann Vonderbrink 2 Lab 1: Gravimetric Analysis of a Metal Carbonate DATA TABLE (to be re‐written in your lab notebook) Mass of crucible Mass of crucible + M2CO3 Mass of crucible + M2CO3 (dried, 1st weighing)
Mass of crucible + M2CO3 (dried, 2nd weighing)
Mass of crucible + M2CO3 (dried, 3rd weighing, if necessary)
Mass of filter paper Mass of filter paper + CaCO3 (1st weighing)
Mass of filter paper + CaCO3 (2nd weighing)
Mass of filter paper + CaCO3 (3rd weighing, if necessary)
ANALYSIS 1. Calculate the mass of dried M2CO3 that you used at the beginning of the lab. 2. Calculate the mass of CaCO3 that you obtained after filtration and drying. 3. Calculate the moles of CaCO3 that you obtained after filtration and drying. 4. Write a balanced chemical equation for the double exchange reaction taking place between M2CO3 and CaCl2. 5. Using the answers to problems 1, 3, and 4, calculate the molar mass of the unknown carbonate, M2CO3. 6. From the calculated molar mass, identify the unknown group 1 metal. 7. Calculate the percent error in the overall M2CO3 molar mass value. 8. Student X did not complete completely dehydrate the original solid. Would you expect their calculated molar mass to be too high or too low and why? 9. Student Y did not add enough CaCl2 to precipitate all of the carbonate. Would you expect their calculated molar mass to be too high or too low and why? Adapted from Laboratory Experiments for AP Chemistry by Sally Ann Vonderbrink 3 AP Chemistry Chemist: ________________________________ Lab 1: Gravimetric Analysis Rubric missing Content Expectations E M A B TOC Table of Contents is complete and up‐to‐date.
Heading includes experiment number, experiment title, date, name, and lab partners. The purpose of the lab is clearly stated and sufficiently describes the experiment. The procedure is given in enough detail for the student to complete the experiment. The initial procedure is only written on the left side of the paper. Procedure used in lab reflects changes made by the class and by the student. All relevant data is collected in this space for future use. Data is recorded in an organized list or table so that it is clear to the reader what the experimenter observed. All significant figures are retained in the data collection and recording process. ‐‐ 2 ‐‐
1
0
‐‐ 2 ‐‐ 1 0 ‐‐ 2 ‐‐ 1 0 4 3 2 1 0 4 3 2 1 0 8 7 6 5 0 1.
Calculates mass of dry M2CO3. ‐‐ 2 ‐‐ 1 0 2.
Calculates the mass of CaCO2. ‐‐ 2 ‐‐ 1 0 3.
Calculates the moles of CaCO3. ‐‐ 2 ‐‐ 1 0 4.
Correctly writes and balances the reaction between M2CO3
and CaCl2. 4 3 2 1 0 5.
Calculates the molar mass of M2CO3. 4 3 2 1 0 6.
Identifies M correctly ‐‐ 2 ‐‐ 1 0 7.
Calculates percent error for the molar mass of M2CO3. 4 3 2 1 0 8.
Correctly determines how the molar mass would be affected by incomplete drying of M2CO3 and gives a reasonable justification for the claim. Correctly determines how the molar mass would be affected by failing to add enough CaCl2 to precipitate all CaCO3 and gives a reasonable justification for the claim. 8 7 6 5 0 8 7 6 5 0 Heading Purpose Initial Procedure Procedure used in Lab/Observations Data Analysis 9.
TOTAL _____ / 58 Grading scale E M A B Exceeds expectations Meets expectations Approaching expectations Below expectations Detailed writing, insightful analysis, flawless grammar, extends scientific concepts, no omissions; approaches college level work Includes all required information, demonstrates understanding of scientific concepts, meets all required grammatical conventions; appropriate for high school level work Minor omissions of information, misunderstandings of scientific concepts, and/or errors in grammar; working toward CCSC standards for high school level work Major errors in scientific concept or grammar, or missing major information; below CCSC standards for high school level work