Experiment 5 Lab Report PDF

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Experiment 5: Percent Water in a Hydrated Salt

Objectives  To determine the percent by mass of water in a hydrated salt. 

To learn to handle laboratory apparatus without touching it.

Abstract  The main objective of this experiment was to use gravimetric analysis to determine the percent by mass of water in a hydrated salt as well as to learn how to properly handle certain laboratory materials without touching it. One of these laboratory materials being the crucible. One must be able to handle the crucible properly with the use of tongs specifically after heating. We hypothesized that if the salt was heated multiple times, the mass would decrease as water evaporates. To complete this experiment, one would measure the mass of water in the hydrated salt, thereafter, heat the sample to drive off the hydrated water molecules, and then again, measure the mass of the remaining salt. One must then repeat this to ensure accuracy. In conclusion, our hypothesis was accepted based on the fact that our calculations show the decrease in mass as our salt was being heated multiple times. Introduction •

As stated previously, the main objective of this experiment was to use gravimetric

analysis, an analytical strategy that depends almost exclusively on mass measurements for the analysis, to determine the percent by mass of water in a hydrated salt. Before experimenting, one must acknowledge the information that was given as well as already interpreted (background information). This background information being that a majority of salts are considered to be

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hydrated. Many water molecules are chemically bound to the ions of salt in its crystalline structure. We hypothesized that if the salt was heated multiple times, the mass would decrease as water evaporates. To test this hypothesis, one would measure the mass of water in the hydrated salt, thereafter, heat the sample to drive off the hydrated water molecules, and then again, measure the mass of the remaining salt. One must then repeat this to ensure accuracy. In this experiment, there are key terms that must be learned in order to fully interpret what is occurring during the experiment. These terms being efflorescent (hydrated salts that spontaneously, without heat, lose water molecules to the atmosphere), deliquescent (salts that readily absorb water), water of crystallization (several water molecules that are chemically bound to the ions of the salt in its crystalline structure), and a anhydrous salt (water molecules are so weakly bound to the ions that heat removes them). With the use of both these terms and numbers calculated one can discover the identity of an unknown sample of hydrated salt. In this case, that unknown sample of hydrated salt being El Salvador.

Materials and Methods 

Please refer to Experiment 5 on pages 85-90 of Laboratory Manual for Principles of General Chemistry by J. A. Beran (10th Ed.). One deviation from the published procedure was that Trial one was calculated accordingly, following the procedure, whereas the second trial was given, us not having to calculate it.

Results/Data

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Calculations Trial 1: B.1. Mass of Hydrated Salt (g) Mass of Hydrated Salt (g)= [Mass of fired crucible, lid, and hydrated salt(g)] – [Mass of fired crucible and lid (g)] Mass of Hydrated Salt = 20.3756g – 19.9024g Mass of Hydrated Salt= 0.473200g B.2. Mass of Anhydrous Salt (g) Mass of Anhydrous Salt(g)= [Final mass of crucible, lid, and anhydrous salt(g)] – [Mass of fired crucible and lid(g)] Mass of Anhydrous Salt (g)=20.1659g – 19.9024g Mass of Anhydrous Salt (g)=0.263500g B.3. Mass of Water Lost (g) Mass of Water Loss (g)= [Mass of hydrated salt (g)] – [Mass of anhydrous salt (g)] 3

Mass of Water Loss (g)= 0.473200g – 0.263500g Mass of Water Loss (g)= 0.209700g B.4.Percent by Mass of Volatile Water in Hydrated Salt (%) Percent by Mass of Volatile Water in Hydrated Salt (%) = [(Mass of water lost(g)/Mass of hydrated salt(g)] (100) Percent by Mass of Volatile Water in Hydrated Salt = [(0.209700 g)/ (0.473200 g)] (100) Percent by Mass of Volatile Water in Hydrated Salt= [0.4431530008] (100) Percent by Mass of Volatile Water in Hydrated Salt= 44.315300085% Percent by Mass of Volatile Water in Hydrated Salt= 44.3153% B.5. Average Percent H2O in Hydrated salt (% H2O) Average % of H2O = ∑x / n Average Percent H2O in Hydrated Salt (% H20) = [89.37] + [42.49]/ [2] Average Percent H2O in Hydrated Salt= [131.86]/ [2] Average Percent H2O in Hydrated Salt= 65.93%

B.6. Standard Deviation of (% H2O)

Standard Deviation of % H2O= Sq rt [1,098.8672]/ [2-1] Standard Deviation of % H2O=Sq rt [1,098.8672] Standard Deviation of % H2O= 33.15% H2O

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B.7. Relative Standard Deviation (% RSD) Relative Standard Deviation (% RSD)

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( s´x ) 100 33.15 ∗100 65.93

Relative Standard Deviation (% RSD) =

Relative Standard Deviation (% RSD) =50.28%RSD Results and Discussion o In this laboratory experiment one can conclude that by doing this experiment a person is able to better grasp the ability to handle laboratory apparatus without touching it. Our hypothesis was that if the salt was heated multiple times, the mass would decrease as water evaporates. As stated previously, to test this hypothesis, one would measure the mass of water in the hydrated salt, thereafter, heat the sample to drive off the hydrated water molecules, and then again, measure the mass of the remaining salt. One must then repeat this to ensure accuracy. This hypothesis was accepted based on the fact that our calculations show the decrease in mass as our salt was being heated multiple times. Several calculations were made to determine the Salt Unknown. That Salt being El Salvador. These calculations include the mass of hydrated salt, the mass of anhydrous salt, mass of water lost, percent by mass of volatile water in hydrated salt, average percent H2O in hydrated salt, standard deviation of (% H2O), and lastly the relative standard deviation. With the use of subtraction, division, and multiplication, these calculations were made. Experimental errors that could have occurred during the laboratory experiment include human error given that there is always of chance of inadequately handling equipment and inaccuracies involving the measurements as well as

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some of the hydrous salt, during heating, could have spattered which in turn removed some of the hydrous salt from the crucible.

Conclusions o The overall goal of this experiment was to use gravimetric analysis to determine the percent by mass of water in a hydrated salt as well as to learn how to properly handle certain laboratory materials without touching it. The objectives were fortunately achieved and in order to achieve this goal, one would measure the mass of water in the hydrated salt, thereafter, heat the sample to drive off the hydrated water molecules, and then again, measure the mass of the remaining salt. We hypothesized that if the salt was heated multiple times, the mass would decrease as water evaporates. Our hypothesis was accepted based on the fact that our calculations show the decrease in mass as our salt was being heated multiple times. Ways the experiment could have been improved is by using a larger crucible. By using a larger crucible, it would decrease the probability that spatter from the hydrated salt could escape the crucible, thus giving us more accurate measurements.

References 1. Beran, J.A. Laboratory Manual for Principles of General Chemistry (10 th Edition), 2008, 87-90. Post-Lab Questions 6

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. a. Because during the cooling of the fired crucible, water vapor condensed on the crucible wall before its mass measurement, the percent water in the hydrated salt would be reported as being too high simply because the crucible’s mass measurement would be high. After heating, the mass of hydrated salt would be low. 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. a. If the oil from one’s fingers is completely burned off then the calculations should be unaffected because, as stated previously, the oil is being completely burned off. However, if the oil was failed to be completely burnt off of the fired crucible the percent water in the hydrated salt would be reported as being too high simply because of the oil causing an increase in the mass of the hydrated salt.

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