Formula of an Unknown Hydrate- 3rd Period

Topics: Water, Chemistry, Water of crystallization Pages: 2 (513 words) Published: August 28, 2013
Lillian Zhang
AP Chemistry
3rd Period
Formula of an Unknown Hydrate
The purpose of the Formula of an Unknown Hydrate lab is to determine the weight percentage of H2O hydrated to an unknown salt and the empirical formula for the hydrated salt. To do this, a massed amount of the hydrate is heated over either a hot plate or Bunsen burner. This causes the water to be removed, leaving the anhydrous salt behind in the dish. The mass of the anhydrous salt, now with the absence of water, is less than the original mass of the hydrate. The difference between the two is the mass of the water removed from the hydrate. As shown in data table 1, the mass of the anhydrous salt is 2.66 grams, and the mass of the water removed is .51 grams. The water of hydration was calculated to be about 2. Based on these measurements, the empirical formula of the hydrate is BaCl2 • 2H2O. This means that for every mole of BaCl2, there is about two moles of water. The weight percentage of water to hydrate is about14.75%. This means that in the total mass of BaCl2 • 2H2O, water makes up about 14.75%. However, labs such as this may not be accurate, since there are many places where error could have occurred. For example, volatile impurities may not have been burned off before the evaporating dish was used, but are removed during the heating process in the experiment. Because of the law of definite proportions, the mass of the anhydrous salt left behind will be correct, since the law states that all elements of a compound will stay the same in mass. On the other hand, the weight percent of water will be too big, because of the excess amount of mass removed from the hydrate. This would cause an increase in the moles of water, which will lead to an inaccurate moles of water to moles of anhydrous salt ratio. Some other sources of error could come from rounding, not heating the hydrate long enough, little bits of crystal sticking to the stirring rod, and simply human errors such as measuring....
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