Archer G11 Partner: Judy Aug Gravimetric Analysis of a Metal Carbonate

Transcription

1 Gravimetric Analysis of a Metal Carbonate Purpose The purpose of this lab is to identify the unknown carbonate. This can be done by finding the mass of the product carbonate and using stoichiometry on that mass to find the molar mass of the unknown carbonate. Once this is done, the molar mass of the unknown carbonate can be determined The significance of this is that it allows chemists to identify unknown substances. Hypothesis The hypothesis is that the unknown carbonate can be identified. By finding the mass of the unknown carbonate, displacing the unknown with calcium, finding the mass of the product carbonate, and using stoichiometry the unknown metal in the carbonate can be identified. Materials: Materials Trial 1 Trial 2 Trial M Calcium chloride (CaCl 2 ) 375 ml Distilled water 600 ml Unknown carbonate sample (M 2 CO 3 ) g g g g precision balance 1 balance 600-mL beaker 6 beakers Bunsen burner 3 burners 30-mL crucible 3 crucibles Crucible tongs 2 tongs Drying oven 1 oven Funnel 3 funnels Filter paper g g g 250-mL graduated cylinder 3 cylinders Wire gauze 3 gauzes Clay pipe stem triangle 3 triangles Ring stand and clamp 3 stands and clamps Timer 3 timers Labeling tags 3 tags Watch glass 3 watch glasses Micro spatula 2 spatulas Procedures: 1.) Put a pipe stem triangle on a clamp of a ring stand 2.) Put a crucible on the triangle 3.) Gently heat the crucible for a minute using a Bunsen burner 4.) Use a tong to transfer the crucible from the triangle to a wire gauze 5.) Let the crucible cools down 6.) Mass the crucible 7.) Add 2 g of unknown carbonate into the crucible 8.) Find the mass of the unknown carbonate plus the crucible 9.) Move the crucible back to the triangle

2 10.) Gently heat the crucible for 2-3 minutes 11.) Let the crucible cool down for about 30 seconds to a minute 12.) Mass the crucible with the unknown carbonate 13.) Repeat step until the mass until the mass is about the same 14.) Pour the unknown carbonate from the crucible to a 600-mL beaker 15.) Add 200 ml of distilled water to the 600-mL beaker 16.) Swirl the beaker to dissolve the unknown carbonate 17.) Add 125 ml of 0.2 M CaCl 2 to the beaker 18.) Wait until the precipitate settles 19.) Weigh a filter paper 20.) Fold the filter paper in a way that will maximize the area for filtering 21.) Put the filter paper in a funnel 22.) Put the funnel in a ring clamp on the ring stand 23.) Put another 600-mL beaker under the funnel 24.) Pour the solution from the first beaker through the filter paper 25.) When nearly all the solutions had been poured, swirl the beaker 26.) Pour the rest through the filter paper 27.) Use some distilled water to wash the beaker 28.) Put the filter paper on the watch glass using 2 micro spatulas 29.) Open the filter paper into a circle 30.) Put a label on the watch glass 31.) Put the watch glass in the drying oven 32.) Wait for a day 33.) Take out the watch glass 34.) Quickly mass the filter paper (using tongs to move it to the balance) 35.) Put it back in the drying oven 36.) Wait for 5 minutes 37.) Repeat step for 2 more times 38.) Repeat step 1-37 for trial 2 and 3 Results: The unknown carbonate was a white powder. When it dissolved, it turned the water into a milky color solution. When CaCl 2 was added, the precipitate was not apparent because the precipitate had the same color as the solution. When the filtrate dried, the filtrate formed a white crumb on the filter paper. There were some black dots in the filtrate of unknown origin.

3 M 2 CO 3 + CaCl 2 CaCO 3 + 2MCl CO 3 2- (aq) + Ca 2+ (aq) CaCO 3 (s) Gravimetric Analysis Table Trial 1 Trial 2 Trial 3 Mass of crucible (g) Mass of crucible + M 2 CO 3 (g) Mass of crucible + M 2 CO 3 (dried) (1st weighing) (g) Mass of crucible + M 2 CO 3 (dried) (2nd weighing) (g) Mass of crucible + M 2 CO 3 (dried) (3rd weighing) (g) Mass of crucible + M 2 CO 3 (dried) (4th weighing) (g) Mass of crucible + M 2 CO 3 (dried) (5th weighing) (g) Calculated mass of M 2 CO 3 (g) Mass of filter paper (g) Mass of filter paper + CaCO 3 (1st weighing) (g) Mass of filter paper + CaCO 3 (2nd weighing) (g) Mass of filter paper + CaCO 3 (3rd weighing) (g) Calculated mass of CaCO 3 (g) Calculated moles of CaCO 3 (mole) Calculated Molar mass of M 2 CO 3 (g/mol) Identity of M 2 CO 3 Na 2 CO 3 Na 2 CO 3 Na 2 CO 3 Calculated percent errors (%) Analysis: Mass of M 2 CO 3 = (Mass of crucible + M 2 CO 3, last trial) (Mass of crucible) Mass of M 2 CO 3 equals to mass of crucible and M 2 CO 3 of the last trial subtract by the mass of crucible Trial 1: = g M 2 CO 3 Trial 2: = g M 2 CO 3 Trial 3: = g M 2 CO 3 Mass of CaCO 3 = [(Mass of filter paper + CaCO 3 (1st weighing) + Mass of filter paper + CaCO 3 (2nd weighing) + Mass of filter paper + CaCO 3 (3rd weighing)) / 3] (Mass of filter paper) Mass of CaCO 3 equals to the average mass of CaCO 3 minus the mass of the filter paper Trial 1: [( ) / 3] = g CaCO 3 Trial 2: [( ) / 3] = g CaCO 3

4 Trial 3: [( ) / 3] = g CaCO 3 Moles of CaCO 3 = (Mass of CaCO 3 ) / (Molar mass of CaCO 3 ) Moles of CaCO 3 equals the mass of CaCO 3 divided by the molar mass of CaCO 3 Trial 1: / = mole CaCO 3 Trial 2: / = mole CaCO 3 Trial 3: / = mole CaCO 2 Molar mass of M 2 CO 3 = (Mass of M 2 CO 3 ) / (Moles of CaCO 3 ) Molar mass of M 2 CO 3 equals to the mass of M 2 CO 3 divide by the moles of CaCO 3 (moles of CaCO 3 = moles of M 2 CO 3 due to the mole ratio) Trial 1: / = g/mol Trial 2: / = g/mol Trial 3: / = g/mol Identity of M = [(Molar mass of M 2 CO 3 ) (Molar mass of CO 3 )] / 2 Identity of M equaled the molar mass of M 2 CO 3 minus the molar mass of CO 3 then divided by 2 Trial 1: ( ) / 2 = = Na = Na 2 CO 3 Trial 2: ( ) / 2 = = Na = Na 2 CO 3 Trial 3: ( ) / 2 = = Na = Na 2 CO 3 Percent Error = (Molar mass of M 2 CO 3 ) / (Molar mass of Na 2 CO 3 ) 100% Percent error is equal to the absolute value of the molar mass of M 2 CO 3 divide by the molar mass of Na 2 CO 3 minus 100% Trial 1: / % = 0.86 % Trial 2: / % = 1.14 % Trial 3: / % = 0.71 % The hypothesis has been proven to be true, according to the results above. The results show that the unknown carbonate is actually sodium carbonate. Since the metal carbonate had been identified correctly, the hypothesis on this lab is correct. The unknown carbonate could have been magnesium carbonate. However, it is shown that the unknown metal in the unknown carbonate is an alkali metal, by the subscript 2 after M. That is the reason that the unknown carbonate had been identified as Na 2 CO 3.

5 Conclusion: The results confirmed the hypothesis with an average of 99.1% accuracy. However, some errors should be corrected in the future to increase the accuracy of the results. Some errors could have occurred that caused the results to be slightly inaccurate. The alkali metal carbonates are hydroscopic so it was heated in order to remove the water. However, the flame intensity used during this experiment could have been too intense thus causing the substance to react with oxygen and form some new compound. The inclusion of oxygen in the product s mass could have caused an increase of molar mass of the unknown carbonate. Not only that, the stirring rod was not used to direct the solution down to the filter paper and so some of the solution might had pass through the side of the filter paper. Therefore, the mass of the CaCO 3 determined would be lower and so the molar mass of the unknown carbonate would become higher. This is because the molar mass was determined by dividing the mass of the unknown carbonate with the moles of carbonate, so if the mass of carbonate decreases, so does the moles of carbonate which causes the mass of the unknown carbonate to be divide by a lower number thus the molar mass increases. To prevent these errors from happening in the future, the flame intensity should always be low when removing water and some equipments such as stirring rods should be use to make sure that the solution is filtered properly.

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