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A gravimetric study, conducted by heating an ionic hydrate salt in a red-hot crucible, reveals that calcium sulfate hydrate contains two waters of hydration (CaSO4·2H2O). by Samantha Student (Ideal lab report) Abstract: A salt is an ionic compound formed by the electrostatic attraction between cations and anions. Sometimes, when salts crystallize, waters of hydration are included within the crystal structure (also called a crystal lattice). The water of hydration is not simply adsorbed onto the surface of the crystals, the water molecules are incorporated right into the crystal lattice throughout, and the lattice water affects the chemical and physical properties of the salt, several forms of hydrate may exist for a salt, each with different properties. Solubility may vary with the type of hydrate. Waters of hydration will decrease the amount of salt available from a given mass of salt sample, so it is important to know the hydration number of a salt before weighing it. We investigated the ionic compound calcium sulfate, CaSO4·NH2O, where N represents an unknown number, in order to determine a value for water(s) of hydration. We heated the hydrate extensively so that the water would be completely driven off and the remaining powder would be anhydrous. By measuring the mass of the water driven off (the weight difference of the crucible + sample, before and after heating) we were able calculate the number of moles of water contained in the hydrate sample, moles H2O = [g water lost by heating / 18g/n ]. The grams of anhydrous sample remaining were used to calculate the moles of ionic compound. N was then simply calculated as a ratio, N= (moles of hydrate water / moles of ionic compound). A loosely covered ceramic crucible containing the sample was heated red-hot for 5 minutes; heating times longer than five minutes were used if the mass continued to decrease with further heating, (and steam continued to be evolved). Adoption of a re-heating procedure ensured total de-hydration. Heating was discontinued on any sample that emitted an acrid odor or smoke (the smoke was clearly different from steam in odor and appearance). The evolution of smoke, and/or an acrid sulfur odor was taken as an indication that the sulfur in the compound was decomposing. Evolution of sulfur was deemed unacceptable because it would clearly lead an erroneously large value for N, (the mass loss of sulfur would be interpreted as water lost in the calculation). Consequently, over-heated samples were not considered in the calculation of N and overheated trials were repeated. Using 3 acceptable samples we discovered that N=2 for calcium sulfate, the actual experimental values obtained were 2.3 ± 0.2. The literature indicated that fractional numbers of hydration were rare, so the hydration number was rounded to the nearest whole number. Introduction: Omit Procedure: Omit Results: Graphs, tables, a summary of the most important data. Values obtained for N Trial# 1. 2.1 2. 2.3 3. 2.5 Mean = 2.3 Standard deviation= 0.2 Discussion Questions 1.Can N be determined for a hydrate by capturing the steam evolved during heating? How would you do this? Can you foresee any problems? (Hint: look up dessicant and condensation). Yes. The hydrogen and oxygen mass in water cannot just disappear (conservation of matter). All the water leaving the crucible can be assumed to be the hydration water present in the crystal. Whether you measure a loss of water from the sample, or measure the water actually given off, you should obtain the same result for the value of N. The difficulty would be in trapping all the steam emanating from the crucible and condensing it into liquid water so that it could be weighed. Perhaps a funnel could be placed over the crucible and a gentle suction applied to suck the steam into a glass tube. The receiver tube would be placed in ice water and the steam would condense to water when it entered the cold tube. The difference in weight between the clean dry tube and the tube with the water would reveal the amount of water the hydrate sample gave off when heated. N could then be calculated in the usual manner (as described in the abstract). The suction would have to be gentle, so that the water did not evaporate from the receiver tube and enter the pump, evaporation could be a problem because water evaporates more readily under a vacuum. Another method might be to suck the steam through a pipe containing a dessicant. A dessicant is a hydrate that has been previously made anhydrous and readily (and quickly) takes up water into its lattice. Anhydrous calcium chloride is a good dessicant. A tube with two ends could be filled with calcium chloride (with cotton plugs preventing the dessicant from falling out). One end would be connected to the funnel over the crucible and the other end to a vaccum pump. Suction would pull the steam into the funnel and through the dessicant tube, and the water would be absorbed. Weighing the dessicant tube before and after the trial would establish the amount of water absorbed. Using the dessicant system, N should remain 2 for calcium sulfate hydrate. A number lower than 2 would mean that the dessicant was not absorbing all of the water. Alternatively, two dessicant tubes could be placed in series, connected by hoses and attached to the suction. If all of the water is absorbed by the first tube, the second tube would not gain mass. If the dessicant in the first tube was not absorbing all of the water, it would be a simple matter to use more dessicant until all the water was absorbed. One problem with both these proposed methods is that the air contains water vapor (air is pretty humid in our climate). Both new methods would capture the humidity in the air and report a false high value for N. Our original method is simplest and probably best. 2. Is there any way to utilize the data from a sample that was overheated? (Hint: Sulfate burns at high temperature to form sulfur oxide gases). Not with the simple system we used in our lab, wherein we simply weighed the crucible before and after heating. Sulfur oxide gases would escape from the crucible and the subsequent loss of mass would be assumed to be a loss of water, yielding a false high value for N. How could one determine whether a loss of mass in the crucible is due to water or to sulfur oxide leaving the sample? This cannot be determined, so with our simple system, so the mass data for water of hydration cannot be recovered from an over-heated sample. Alternatively, if the emitted steam and sulfur oxide gases were trapped as described in question #1, the data might be recoverable, as long as the dessicant does not trap the sulfur oxide gases. The dessicant tube could be used to trap water only, even if the sulfate in the hydrate was decomposing. The sulfur oxide gas would pass through the dessicant and the sulfur oxide would not be counted as an emitted mass, thereby yielding a correct value for N, even in the case of severe over-heating. **The preceding lab report would garner a Chem II student an “A”. Certain key attributes make the report exemplary. Look for the following elements in the sample report--make sure that they are present in your report. Title: 1. Are all the important key words mentioned in the report present in the title? 2. Is the title a simple, clear statement (hopefully only one sentence long)? 3. Does the title state important conclusions from the work? 4. Does the title communicate how the work was done (experimental method)? Abstract: 1. Does the abstract contain brief elements of introduction, method, results and conclusion? 2. Does the abstract adhere to a strict one-page limit? 3. Are important observations mentioned? 4. Are important experimental errors described? 5. Do the last few lines clearly state a conclusion? 6. Is the reason for doing the work explained? 7. Are the important results summarized? 8. Is the level of detail sufficient to let the reader grasp exactly what the work is about? 9. Is the abstract comprehensible on it’s own, without the other sections of the report? 10. Is the audience an informed classmate? (not the instructor or a technically learned person). Results: 1. Are tables or graphs used effectively and appropriately? 2. Are all the important results summarized? 3. If replicate measurements are involved, is a mean and standard deviation reported? 4. If linear regression is used, is the linear equation and correlation coefficient reported? 5. Are axes appropriately scaled on graphs? 6. Is excessive detail and reporting of non-essential data avoided? 7. Is there a sample calculation? Discussion 1. Is the audience an informed classmate? (not the instructor or a technically learned person). 2. Does the discussion adhere to a strict one-page limit? 3. Is there reasonable and creative speculation when needed? 4. Are the arguments cogent and convincing? 5. Were the “hints” used to guide information searches? 6. If the discussion question requires a calculation, is it clearly shown? 7. Is there an in-depth, informed discussion, based upon supplemental reading?