Determining the Number of Moles of Water of Crystallization Associated With One Mole of Copper (II) Sulfate in a Hydrate CuSO4 * xH2O (s).

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INTRODUCTION Many salts that have been crystallized from water solutions appear to be perfectly dry, yet when heated they discharge large quantities of water. An example can be hydrated copper (II) sulfate. Exactly this salt is used in the described experiment. RESEARCH QUESTIONWhat is the number of moles of water of crystallization associated with one mole of copper (II) sulfate, in the hydrate CuSO4 * xH2O (s)? The independent variable in this experiment is sample thermal treatment ( heating and cooling) and the dependent variable is the number of moles of water of crystallization. A number of other variables can affect the final result of the experiment: whether the crucible was clean and dry before filling it with the salt, whether the sample was heated too strongly and cooled too long before weighting. HYPOTHESIS In the hydrate copper (II) sulfate one mole of salt is believed to be combined with five moles of water. Bearing this in mind the following hypothesis was made: If the hydrate is heated until there is no further loss in weight, five moles of water will be lost from one mole of salt. THEORY When certain ionic solids crystallize from aqueous solutions, they combine with a definite amount of water, which becomes a part of the crystalline solid. Salts that contain water as part of their crystal structure are called hydrates (or hydrated salts) and the water in the crystal structure is called the water of hydration (or water of crystallization). When the water of hydration is removed from the hydrate, the salt that remains is said to be anhydrous. For a hydrate, the number of moles of water present per mole of salt is usually some simple, whole number. The formula for a hydrated salt is usually written as MN . x H2O, where MN means a salt, x indicates the number of water molecules that are associated with one molecule of salt and the dot means that water molecules are rather loosely attached to the salt. In this experiment, the mass of the water driven off, as well the mass of the salt remained was determined. Then using the molar mass of CuSO4, the amount in moles of water and the anhydrous salt was calculated. Finally given the mole ratio, the formula of the hydrated salt was founded. The essential formulae for this experiment were:n= m : M n- number of molesm- mass of a substance [g]M- molar mass [g/mol]% error= I(A%-E%) : A%I * 100%A%- actual percentage of H2O in a hydrateE% - experimental percentage of H2O, in a hydrateII - absolute valueMATERIALSspectaclescrucible (50 ml)crystals of hydrated copper (II) sulfatesupport standburnerbalance (accuracy 0.05g)stirrertongstest tubepipetteMETHODDuring heating the spectacles should be worn.Hot crucibles should be carried with tongs, not with hands.The crucible should be cooled before weighting. 1.The empty, clean and dry crucible was taken and weighted (the tare button was used before placing the crucible on the balance). The result was recorded in a raw data table.2.The crucible was filled with a little amount of a hydrated copper sulfate (in a way, that the salt covered with a thin layer the bottom of a crucible).3.The crucible with the hydrated salt was weighted without using the tare button and the result was recorded in a raw data table.4.The crucible was placed on a support stand. The burner was lighted (a very light, almost transparent flame was set) and placed under the crucible. (see fig.1)5.The crucible was heated for about 10 minutes and the salt was stirred for the whole time. The observations were recorded.6.The crucible was taken with the tongs from the burner and left to cool.7.When the crucible was cool enough to hold it in the hands, it was reweighed (the tare button was used) and the result was recorded in a row data table.8.The crucible was heated again for about 8 minutes, cooled and reweighed. The result was recorded.9.The crucible was heated again for about 2 minutes, cooled and reweighed. The result was recorded. There was no further loss in...
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