Determination of a Chemical Formulae lab write up: Experiment 1 PDF

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This is the lab write up for Experiment 1....

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Determination of a Chemical Formulae Experiment 1

Abstract In this experiment, different masses of Iodine and Zinc were reacted to investigate the Law of Constant Composition and Conservation of Mass from the Zinc Iodide produced. Zinc Iodide was synthesized by reacting a specific mass of Iodine with excess Zinc in a slightly acidic solution. The empirical formula for Zinc Iodide was found to be ZnI2, which was supported by the data as the Zinc and Iodine reacted in a 1:4 ratio regardless of initial masses reacted, validating the Law of Constant Composition. The experiment also validated the Law of Mass Conservation as the initial mass of Zinc and Iodine reacted was very close to the mass of the Zinc Iodide produced, with a percent error of 4.2% due to experimental error. Experimental Section Materials  20 to 30-mesh granular zinc metal  solid iodine  6 M acetic acid  distilled water  glazed weighing paper  desiccator (plastic bags with silica gel desiccant)  copper wire electrodes  9 V battery

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alligator-clip electrode connectors 100 mL beaker 200 mL beaker 50 mL beaker 25 mL graduated cylinder spatula balance glass stirring rod heating plate

Procedure The mass of a 100 mL beaker and a piece of glazed weighing paper was recorded (+/- 0.1 g). With the spatula, 2.006 g of Zinc metal was measured on glazed weighing paper and then transferred to the beaker. The mass of the beaker and the zinc was recorded, and the mass of the Zinc recorded by difference. This method was repeated with 2.744 g of Iodine, in the same 100 mL beaker with the mass of the added Iodine recorded by difference. 9 drops of 6 M acetic acid were added to 15 mL of distilled water in a graduated cylinder, and then 10 mL of this solution were poured into the 100 mL beaker containing the Zinc and Iodine. The contents were stirred with a glass-stirring rod. While waiting for the reaction to complete, one boiling stone was added to a tall 200 mL beaker, and the mass of the beaker containing the stone was recorded. After the reaction completed (visible due to the clear color of the solution), the liquid contents were decanted into the 200 mL beaker. The solid Zinc that remained was rinsed three times with 1 mL of the acidified water solution, and then the liquid was decanted into the 200 mL beaker. The solid Zinc was rinsed again with the remaining acidified water, but this time the liquid was decanted into a 50 mL beaker. The 100 mL beaker containing the solid Zinc was dried on a hot plate until the Zinc no longer clung to the beaker, and no condensation was visible. After cooling to room temperature, the mass of the dry Zinc and beaker was recorded, and the mass of the

Zinc recorded by difference. The solution in the 50 mL beaker was disposed of in the hood. The 200 mL beaker containing the Zinc Iodide solution was heated for 25 minutes to evaporate the solvent and stirred with the stirring rod to prevent burning. Once the sound of crackling stopped and the solute appeared off white, the beaker was removed from the hot plate and left to cool for 1 minute. After putting silica gel desiccant in the bag, the beaker was placed in the bag and allowed to sit for 10 minutes. The mass of the beaker including the beaker and boiling stone was recorded, and the mass of the Zinc Iodide found by difference. A small sample of Zinc Iodide was scooped out of the beaker with the spatula and placed on a watch glass. A few mL of distilled water were added to the glass to dissolve the compound. The solution was electrolyzed by placing the two copper-wire electrodes, connected to the battery by alligator-clips, in the solution for 2 minutes. The solution was then disposed of in the waste hood.

Results and Discussion Results Initial mass of Zinc: 2.006 g Initial mass of Iodine: 2.744 Mass of dry unreacted Zinc: 1.294 g Total mass of reacted Zinc and Iodine: 3.456 g Mass of Zinc Iodide produced: 3.601 Observations: Mix of Zinc and Iodine in solution

Zinc Iodide solution boiling

Zinc Iodide with electrodes

When the 10 mL of the solution were poured in, the solution immediately became a yellow orange color. After 5 minutes, the solution was still at room temperature but appeared to be a darker orange/brown. After 8 minutes, the solution heated up and it was apparent that the Iodine was dissolving. After 11 minutes, the solution was hot and appeared dark brown. At 14 minutes, the solution got lighter and appeared to be more orange. At 16 minutes, the solution was yellow and warm, and finally at 18 minutes it was clear and a little warm. Water vapor came out of the beaker and the solution turned more yellow. The yellow darkened for a while until it appeared to be a pale yellow liquid with a white slush substance in the middle. The solid made a crackling noise, and after 10 minutes, there was an off-white chalky substance. A dark orange/brown color identified as Iodine was attracted to the positive electrode while a white substance identified as Zinc was attracted to the negative electrode.

Calculations: a) Mass of reacted Zinc and Iodine: ( 2.006 g Zn+2.744 g I )−1.294 g Zn =3.456 g b) Percent Error: (3.456 – 3.601)/3.456 = 4.2 %

c) 2.744 g I(s) ×

1 mole I =.022 mole I ( 126.9 gI)

d) 2.006 g Zn(s) – 1.294 g Unreacted Zn(s) = 0.71 g Zn 1 mole Zn = .011 mole Zn 0.71 g Zn × 65.38 g Zn

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Empirical formula for Zinc Iodide: ZnI2 Graphs:

Discussion & Error Analysis According to the data, the empirical formula for Zinc Iodide is ZnI2. The observation that there was no visible Iodine leftover in the solution after the reaction supported the assumption that all the iodine reacted with the Zinc. The relationship between the mass of Zinc reacted and the mass of Iodine reacted, as illustrated in Graph 1, demonstrates that Zinc Iodide has a 1:4 mass ratio. Two moles of Iodide react with every one mole of Zinc, and Iodine is also twice as heavy as Zinc according to their weight (g) per mole. Thus, the ideal slope for Graph 1, with perfect data, would be 4.0, and the intercept would be zero. For every gram of Iodine reacted, ¼ of that mass in Zinc is reacted. This relationship is also demonstrated in Graph 2, as it illustrates that the ratio is constant independent of the amounts of reactants being combined. If the group data were perfect, the intercept in Graph 2 would be 4.0 and the ideal slope would be 0, as any weight of Zinc would be multiplied by 4 to find the weight of reacted Iodine. Graph 1 and Graph 2 illustrate the validity of the Law of Constant Composition. Graph 3 supports the conservation of mass demonstrates in the experiment. With perfect data, the slope would be 1 and the intercept would be zero, because the mass of the reacted Zinc and Iodide must be equal to the mass of the Zinc Iodide produced, as matter can never be created or destroyed. The slight deviation in the actual results to this law is due to the errors in the experiment. The main error in this experiment was that the mass of the Iodine recorded initially to transfer into the 200 mL beaker included the mass of the weighing paper. Thus, when the Iodine was transferred to the beaker and the difference recorded, the mass was under 3.0 g at 2.744 g. Also, it is likely that some of the Zinc Iodide product may have been burned while precipitating in the solution over the hot plate. This may explain why the mass of the Zinc Iodide that was produced (3.601 g) was greater than the total mass of reacted Zinc and Iodine (3.456 g). The percent error of 4.2% is low, so the experimental results still mostly support the Law of Conservation of Mass. Based on the experiment, the statement that “The properties of a chemical compound are the averages of the elements that make up the compound” is false. The individual properties of the elements before the reaction were that they were both solid, but the Zinc was metallic and granular while the Iodine was dark and not granular. The properties of the compound were very different from either of those observations, as it was light grey, chalky substance. When the Zinc Iodide was decomposed through the use of electrodes, the elements separate and mostly returned to their aqueous properties, where Iodine was a rust-brown color and the Zinc a whitegrey color. The Iodine was attracted to the positive electrode because it is electronegative with a negative charge and the Zinc was attracted to the negative electrode because it has a positive charge....