Gravimetric Determination: Standards
Claire Miller Partners: Kyle O’Donnell, Evan Rudomen Chem 315 - L03 January 30, 2013
Data
Solution 1 was made using 20.002 g of the unknown sample 1 in 100 mL of water. Table 1. Weights of Cathode Before and After Electrolysis in Solution 1
Initial Weight of Pt Cathode
Trial 1
Trial 2
Trial 3
9.975 g
9.803 g
9.798 g
10.339 g
10.244 g
Final Weight of Pt Cathode (w/ Cu) 10.497 g
Solution 2 was made using 19.999 g of the unknown sample 2 in 100 mL of water. Table 2. Weights of Cathode Before and After Electrolysis in Solution 2
Trial 1
Trial 2
Trial 3
10.428 g
10.429 g
10.422 g
Final Weight of Pt Cathode (w/ Cu) 10.802 g
10.821 g
10.806 g
Initial Weight of Pt Cathode
Results and Discussion
From the collected data of the weights of the cathode before and after electrolysis, the amount of copper in each solution could be determined. The volume of each solution was 100 mL and each trial used 10 mL of the stock solution, so assuming that the solutions were homogeneous, the amount of copper found in each trial would be 1/10 of the total copper in the stock solution. From this information, the percent of copper in each sample could easily be determined since the total weight of the sample added to the stock solution was known. These results can be seen in tables 3 and 4 below. Table 3. Amount of Copper in Solution 1 and Percent Copper in Sample 1
Trial 1
Trial 2
Trial 3
Amount of Copper
0.522 g
0.536 g
0.446 g
Percent Copper in Sample 1
26.1%
26.8%
22.3%
Table 4. Amount of Copper in Solution 2 and Percent Copper in Sample 2
Amount of Copper
Trial 1
Trial 2
Trial 3
0.374 g
0.392 g
0.384 g
Percent Copper in Sample 2
18.7%
19.6%
19.2%
Using this information, I determined the percent of copper in sample 1 to be 25.1% ± 2.42, and the percent of copper in sample 2 to be 19.2% ± 0.451. The standard deviation in the percent of copper in sample 1 is large since trial 3 does not appear to be as precise as trials 1 and 2. The trials in sample 2 however were much more precise, hence the small standard deviation. The method of gravimetric analysis is helpful when the initial amount is unknown. As long as the reactions that have taken place are known, it is possible to work backwards from the amount of product obtained to get the amount of the starting material. In this case, the sample was a mix of copper and other unknown compounds and the information desired was the percent of copper in the original mixture. Using a known amount of sample, electrolysis can separate out the copper to figure out how much copper was in that amount of the sample. Since the actual amount of copper was unknown, it is not possible to obtain the accuracy of this data. However, it was said in the exp erimental procedure that the amount of copper in each sample should be between ~4 - 6 g. Sample 1 was determined to have just above 5 grams and sample 2 was found to have just below 4 so these values fall approximately in this range. The precision of the trials can be calculated without knowing the actual value, since it is just how close the trials were to each other. It is clear from looking at the results, that the trials run on sample 2 were much more precise than sample 1. In sample 1, the first two trials were much more precise than the last. The percent relative standard deviation can be calculated for each sample to illustrate this. For sample 1, the %RSD = 9.64, while the %RSD = 2.35 for sam ple 2. Data and Calculations
10 mL of the 100 mL solutions were used in each trial, so assuming the copper was uniformly distributed throughout the solution, the amount of copper found in each trial would be 10% of the total copper in the sample. Sample 1 Trial 1:
Trial 2:
Trial 3:
Mean:
Standard Deviation:
%RSD:
Confidence Interval:
Sample 2 Trial 1:
Trial 2:
Trial 3:
Mean:
Standard Deviation:
%RSD:
Confidence Interval:
Error Discussion
One significant source of error could be if the cathode wasn’t dry either before the electrolysis or after when it was weighed. This could effect the results is one of two ways. If the cathode wasn’t dry for the initial weighing, then the initial weight would be too high and it would seem like less copper plated out of the solution. If the cathode wasn’t dry when it was weighed after the reaction, it would have seemed like there was more copper in the solution than there actually was, giving an incorrect percent copper. This can be corrected by drying the cathode for longer and then waiting approximately five minutes after weighing to weigh again to see if there was still any water on it. Another significant source of error could be in the solution wasn’t homogeneously mixed. An assumption was made that finding the amount of copper in one-tenth of the solution and multiplying it by 10 would give the total amount of copper in the solution. However, if the solution was not homogeneous then each 10 mL sample would contain different amounts of cop per. This would not only make the accuracy wrong, but the precision would also be off. While it is hard to prove that the solution is definitely homogeneous, ensuring that it is mixed well is a good way to limit the possibility of this error. A third significant source of error could be if the copper interacted in some way with one of the solvents, causing less of it to plate out. This would result in it appearing that there is a lower percentage of copper in the original sample. A way to ensure this isn’t happening would be to perform a standard addition with the copper and the solvent and check that the amount of copper in the solution is increasing by the correct amount. If it is not, calculate what the correction factor should be to accurately find the amount of copper in the samples.
