CHEM 111 Experiment 5 - lab report PDF

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Destiny Cambero CHEM 111 – Farnum MTWR 9:40 AM 07/02/18

Experiment 5: Percent water in a hydrated salt Conclusion: My unknown was #3 Na2CO3, the overall mass decreased when heated, its percent average in hydrated mass was 15.21%. My standard deviation was 0.311%, my relative standard deviation was 2.05%, and my error was 4.7%.

Experiment 5: Percent water in a hydrated salt-Discussion The goals of this experiment were to find the mass percent of water in a given salt by using gravimetric analysis and to learn how to handle laboratory equipment without touching the sample with our hands. Gravimetric analysis is just a way to interpret the data solely on mass measurements. Each student was given an unknown hydrated salt and was expected to find the mass percent of water in their sample with the technique of heating resulting in an anhydrous salt. A hydrated salt is a salt that contains water, an anhydrous salt is one that lacks it. I hypothesized that the mass of the original sample would decrease after heating, this decrease in mass would give us our water measurement from the sample by subtracting the initial hydrated salt from the final anhydrous salt. For this experiment, there are many steps that seem very tedious and unnecessary, however each step is important in obtaining an accurate result. The steps that seem excessive tend to be the most important. Heating and cooling take up most of the time during this experiment, the first step requires heating the crucible to ensure that it is clean and free of impurities. A sufficient heating time is about five minutes, I discovered that a ten-minute cooling period seemed to work for me so I tried to keep this consistent throughout my experiment. We were then expected to measure out about three grams of our salt, I always tried to remain just slightly under three, never more. Then we would heat the sample to remove the water from the sample. We were expected to leave the lid slightly open to allow the water molecules to escape while heating and close the lid when cooling to prevent water from entering the sample. I would sometimes forget to either lift or close the lid at times, this could have caused water to not escape or re-hydrate the sample causing some error. After weighing we were expected to heat one more time for two minutes to see if we had removed all of the water from the sample. Many would most likely skip this step however this could result in an inaccurate water percentage so it is necessary to reheat and reweigh to check how well the sample was heated. We would then perform a second trial and repeat this process from start to finish. As far as errors go I may have created error by not opening and closing the lid of the crucible when necessary. I also accidentally dropped the crucible while weighing it, although the sample did not spill water could have rehydrated the sample. I also touched the crucible at one point but I was heating so this should not have caused any error. Based on my hypothesis my result was correct, the initial mass of the hydrated salt decreased after being heated into an anhydrous salt. However, I only performed two trials for my experiment instead of the initial three due to a lack in time. In this experiment, we calculate the standard deviation to see how reliable the results were based on how precise the average is and by measuring the random error. This is calculated by taking the square root of the sum of the initial value subtracted by the final value squared, divided by the total number of samples subtracted my one. A good standard deviation has many points/trials, however with the two I performed I acquired a standard deviation of 0.311%. A smaller SD represents a closer average, being that my SD is lower than 1% it is an okay result. However, an extra trial could have made a difference in this data. As for the relative standard deviation, this is used to test variability. This is calculated by taking the SD and dividing it by the average and multiplying it by 100. My RSD

was 2.05%, as mentioned earlier these calculations work best when there is more data to compare to, based on my results there is some error but nothing out of the ordinary. The major results in this experiment was that of the mass percent of water obtained in the sample, the standard deviation, the relative standard deviation, and my percent error. I hypothesized that the mass of the initial sample would decrease which was correct. Both my standard deviation and my relative standard deviation were less than 10%, which shows that my data is both accurate and precise. This is achieved by following the protocol to avoid contamination and error. In order to perform Gravimetric analysis, we need accurate readings of the data, this ensures that our measurements have as little error as possible. These results were as I expected because although I made mistakes throughout my experiment, there wasn’t anything drastic that would have resulted in any major errors. My error was 4.7% which was under 10%, this also shows that my data is accurate. My results ended up being good for my sample. My unknown salt was Na2CO3, the formula was Na2CO3 * H2O, this means that my sample is a monohydrate. Conclusion In conclusion, my unknown was #3 Na2CO3, the overall mass of my sample decreased when heated, its percent average in hydrated mass was 15.21%. My standard deviation was 0.311%, my relative standard deviation was 2.05%, and my percent error was 4.7%. Errors Errors that occurred throughout my experiment was the position of the crucible lid when heating and cooling. Sometimes I would forget to lift or close the lid and this error could have caused water to stay in or rehydrate the sample, increasing the sample mass. Another error that had occurred was that I dropped the crucible with the sample and the lid fell off. The time it took me to recover the lid without touching it with my hands could have caused water to rehydrate the sample. Two errors that could have occurred could have been that I either touched the sample with my hands or placed it on the counter contaminating it. Another error could have been that I overheated the sample causing it to either decompose or to cause the salt to pop. This would have altered the water content and mass of the sample. Experiment 5: Percent water in a hydrated salt-Post lab questions 1. Part A.1. During the cooling of the fired crucible, water vapor condensed on the crucible wall before its mass measurement. The condensation did not occur following thermal decomposition of the hydrated salt in Part B. Will the percent water in the hydrated salt be reported as being too high, too low, or unaffected? Explain. If the water condensed on the walls of the crucible before its mass measurement this would give the crucible a higher overall initial mass. In part B the salt was added and this condensation did not occur, this would give an overall lower mass reading the second time because the initial water and the water within the sample will both evaporate. Therefore, because the water was present the first time and not the second time for the second reading the percent water will be higher than it should. This is because there was water initially present and this water was not taken into account for the final measurement because it was not present in the salt itself.

2. Part A.1. The fired crucible is handled with (oily) fingers before its mass measurement. Subsequently, in Part B.1, the oil from the fingers is burned off. Will the percent water in the hydrated salt be reported as being too high, too low, or unaffected? Explain. The result for this case would be the same as the previous question with the additional water. The oil will add additional mass that will go away with the second measurement after burning, this would give a result that a larger mass percent of water had evaporated when it was actually additional mass given off by the oil. This will result in an inaccurate reading of how much water had actually evaporated from the sample. 3. Part A.1. The crucible is handled with (oily) fingers after its mass measurement but before the -3g sample of the hydrated salt is measured (Part A.2). Subsequently, in Part B.1, the oil from the fingers is burned off. Will the percent water in the hydrated salt be reported as being too high, too low, or unaffected? Explain. The crucible mass would be correct but the mass of the -3g salt will be heavier that its actual mass due to the addition of the oil. When heated the oil will burn off with the addition of the water that will evaporate from the salt, this would make it appear that there was more water mass in the sample than there actually was present. Therefore, the percent of water present in the sample would be recorded as being too high due to the additional weight of the oil. 4. Part A.2. Suppose the original sample is unknowingly contaminated with a second anhydrous salt. Will the reported percent in the hydrated salt be too high, too low, or unaffected by its presence? Explain. An anhydrous salt is a salt that lacks water and is dry. Since we are adding an additional anhydrous salt, this means that we are adding more dry mass but not more water. Since we are adding more of a substance lacking water we will have a lower percent of water to be present than there should be. 5. After heating in Part A.1, the crucible is set on the lab bench, where it is contaminated with the cleaning oil used to clean the lab bench, but before its mass is measured. The analysis continues through Part B.1, where the mass of the anhydrous salt is determined. While heating the cleaning oil is burned off the bottom of the crucible. Describe the error that has occurred; that is, is the mass of the anhydrous salt remaining in the crucible reported as being too high or too low? Explain. Since the crucible was set on the bench and contaminated with the cleaning oil, the mass will be greater than it should be causing an error. When heating the anhydrous salt the mass of the percent water and the oil will evaporate, however it will not evaporate as fast due to the fact that we are still burning for the same amount of time. This will result in the mass of the remaining anhydrous salt to be too high due to the cleaning oil. 6. Part B.1. the hydrated salt is overheated and the anhydrous salt thermally decomposes, one product being a gas. Will the reported percent water in the hydrated salt be reported too high, too low, or be unaffected? Explain. Although we want to remove all the water from the sample we do not want to remove the sample itself. Removing all the water is beneficial, however the difference in the sample mass would be too large in order to get accurate results. So the remaining salt mass would be too low resulting in a high percent of water for the remaining sample due to its decomposition.

7. Part B.1. Some of the hydrated salt spatters out of the crucible because of a too rapid heating process. Will the reported percent of water in the hydrated salt be reported too high, too low, or unaffected? Explain. If some of the hydrated salt spatters out this will alter the mass of the salt and the mass percent of water. The final weight of the anhydrous salt compared and that of the hydrated salt will differ too much in comparison and there will be less water by mass than there should have been. This will result in the percent of water in the hydrated salt to be too high compared to the final measurement of the anhydrous salt after heating. 8. Part B.2. Because of a lack of time, Bill decided to skip this step of the Experimental Procedure. Will his haste in reporting the “percent H2O in the hydrated salt” likely be too high, too low, or unaffected? Explain. If Bill skips this step, he does not ensure that he has removed all of the water content in the salt. This could alter his result depending on how well he heated his sample. If there is water still present in his sample, then his percent of water will be lower than expected, giving him an inaccurate result and error....