Ms. Sonderleiter AP Chemistry Name: Date: Lab #4 Gravimetric Analysis of a Metal Carbonate (adapted from Flinn Scientific ChemFax, 2005) Background: In this experiment, an unknown alkali metal carbonate, M 2 CO 3, is analyzed to determine the identity of the metal. A known amount of the soluble unknown carbonate is dissolved in water to dissociate the compound into ions (Equation 1). M 2 CO 3 (s) 2M + (aq) + CO 3 2- (aq) Equation 1 When a solution of calcium chloride, CaCl 2, is added to this metal carbonate solution, a precipitate of calcium carbonate forms (Equation 2). Ca 2+ (aq) + CO 3 2- (aq) CaCO 3 (s) Equation 2 The overall reaction represents a double-displacement reaction with a precipitate formed (Equation 3). CaCl 2 (aq) + M 2 CO 3 (aq) CaCO 3 (s) + 2MCl(aq) Equation 3 The precipitated calcium carbonate is then filtered, dried, and weighed. The molar mass of the alkali metal carbonate can be determined using stoichiometry. Purpose: To determine the identity of an alkali metal carbonate using gravimetric analysis of a double-displacement precipitation reaction. Materials: Unknown metal carbonate Calcium chloride solution, 0.2 M Distilled water Ring stand Filter funnel Stirring rods Bunsen burner Crucible Electronic balance Beakers, 400-mL Drying oven Filter paper Clay triangle Tongs Watch glass
Procedure: Dehydrating the Alkali Metal Carbonate 1. Prepare the crucible by heating for approximately 1 minute, allowing it to cool, and finding its mass. 2. Add approximately 2 g of the unknown carbonate to the crucible and find the combined mass of the crucible and carbonate. 3. Heat the crucible and carbonate form 2-3 minutes. Cool and mass. 4. Repeat step 3. Conducting the Precipitation Reaction 5. Add the crucible contents to a 400-mL beaker. 6. Add about 125 ml of 0.2 M CaCl 2 and stir. Let the precipitate settle (5 min). 7. Filter out the precipitate using a piece of massed filter paper. 8. Place the filter paper on a watch glass and place in a drying oven for 10-15 minutes. 9. Remove and find the mass of the sample. 10. Break up the solid and return to the drying oven for 5 minutes. 11. Find the mass of the sample. Data/Observations: Mass of crucible Value Mass (g) Mass of crucible + M 2 CO 3 Mass of crucible + M 2 CO 3 (dried) Mass of crucible + M 2 CO 3 (dried 2 nd time) Mass of filter paper Mass of filter paper + CaCO 3 (1 st time) Mass of filter paper + CaCO 3 (2 nd time) Observations:
Calculations: 1. Determine the mass of the hydrous alkali metal carbonate. 2. Determine the mass of the anhydrous alkali metal carbonate. 3. Determine the mass of the final dried calcium carbonate product. 4. Using the mass of dried calcium carbonate, determine the number of moles of alkali metal carbonate present in the original sample. 5. Using the moles of alkali metal carbonate present and the mass of the alkali metal carbonate, determine the molar mass of the sample. 6. Using your value for #5 and the actual value, determine your percent error.
Conclusion Questions: 1. Why is the alkali metal carbonate heated before it was reacted with the calcium chloride solution? 2. Why is calcium chloride solution used instead of a solution of potassium chloride? 3. Why is the calcium carbonate removed from the oven after 10-15 minutes of heating, crushed, and then returned to the drying oven? 4. Based on the calculation above, what is the identity of the unknown alkali metal carbonate? Why? 5. What are some potential sources of error in this experiment? Explain.
6. How could you improve this experimental procedure to reduce the likelihood of error? Explain. 7. If you accidentally spilled some of the alkali metal carbonate before mixing it with the calcium chloride solution, how would the calculated value of molar mass be affected? Explain. 8. If tap water were used in place of distilled to dissolve the dried alkali metal carbonate, how would the value of molar mass be affected? Explain.