4th Hour

September 30, 2010

Overall this was a great lab. The background was the only section that I took off points. I felt that ideas needed to be connected a little better so the paragraph had better flow.

Determination of a Chemical Formula

Purpose –

The objective of this lab is to find the formula of copper chloride hydrate, which has a general formula of CuxCly zH2O. This will be done by finding the masses of copper, chlorine, and water in a sample of the substance. The water will be evaporated off and the chlorine will be filtered out.

Background –

The law of conservation of mass states that the mass before a reaction is the same as the mass after a reaction. Because of this law, the massesof the components of copper chloride hydrate can be determined after performing various experiments on the substance. The composition of a pure substance, regardless of its source, is the same, as stated by the law of definite proportions. This means the empirical formula is the same for every sample of copper chloride hydrate. The law of multiple proportions states that elements combine in small, whole number ratios. This will be helpful in finding the chemical formula of copper chloride hydrate. A chemical formula is composed of symbols and corresponding numerical subscriptsthat give the proportions of the atoms of the elements in a substance. The empirical formula is the simplest ratio of these atoms. The empirical formula can be determined by calculating the mole ratio of the substance.

A mole is a unit in chemistry that represents an amount of molecules, ions, or atoms. By definition, a mole is the number of atoms in 12 grams of 12C. The number of atoms in 12 grams of12C is approximately 6.02 x 1023 (called Avogadro’s number). The mass of an element is equal to the number of moles of that element. For example, hydrogen has a mass of one gram, which is equal to one mole of hydrogen, while, oxygen has a mass of sixteen grams, which is equal to one mole of oxygen.

This is then used todetermine the chemical formula of a substance. First,determine the masses of the atoms of the elements in the substance, and then convert it to moles. The ratio of the moles of each atom in the substance is the mole ratio, which is then used to determine the chemical formula. The mole ratio of the atoms becomes the subscripts that correspond to that element. Take water, for example. There are two moles of hydrogen for every one mole of oxygen in water. Therefore, the chemical formula is H2O. By determining the masses of the components of the substance, it is possible to calculate the chemical formula.

Procedure –

First, find the mass of the water in the sample by massing the sample before and after heating it. Heating the sample causes the water to evaporate off. Then, combine the sample with water and aluminum wire. After adding hydrochloric acid, remove the aluminum wire, leaving metallic copper in a solution with aluminum salt. Next, use a funnel and filter paper to filter out the aluminum salt, which leaves only copper. Finally, after adding ethanol to the copper and allowing it to dry under a heat lamp, mass the final substance.

Data –

Raw Data

Mass of Crucible / 12.972 g
Mass of Crucible and Hydrated Sample / 13.968 g
Mass of Crucible and Dehydrated Sample / 13.762 g
Mass of Empty Watch Glass / 18.391 g
Mass of Filter Paper / 0.954 g
Mass of Watch Glass, Filter Paper, and Copper / 19.728 g

Inferred Data

Mass of Hydrated Sample / 0.996 g
Mass of Dehydrated Sample / 0.790 g
Mass of Copper / 0.383 g
Number of Moles of Copper / 0.00603 mol
Mass of Water Evolved / 0.206 g
Number of Moles of Water / 0.0114 mol
Mass of Chlorine in Sample / 0.407 g
Number of Moles of Chlorine / 0.0115 mol
Mole Ratio, Chlorine: Copper in Sample / 2:1 (1.91:1)
Mole Ratio, Water : Copper in Hydrated Sample / 2:1 (1.89:1)
Formula of Dehydrated Sample / CuCl2
Formula of Hydrated Sample / CuCl2 2H2O

Actual Values

Mass of Copper / 0.371 g
Mass of Water / 0.210 g
Mass of Chlorine / 0.414 g
Formula of Dehydrated Sample / CuCl2
Formula of Hydrated Sample / CuCl2 2H2O
Percent Error for Copper / 3.23%
Percent Error for Water / 1.90%
Percent Error for Chlorine / 1.69%

Observations –

The hydrated sample was a blue-green color and it was in clumps. Heating the sample caused the color to change to brown, and it had an unpleasant odor. After it cooled, it still looked brown and was clumped. Adding the aluminum wire caused the mixture to fizz and bubble. It also made a soft popping noise because of the fizzing. The color turned from blue to a colorless, foggy mixture. The copper that attached to the wire was a bronze color and it attached to the wire in clumps. The filtered solution was still colorless and foggy. At the conclusion of the experiment, the copper was a bronze color and it was in clumps.

Calculations –

Mass of Hydrated Sample

Mass of Crucible and Hydrated Sample – Mass of Crucible = Mass of Hydrated Sample

Mass of Dehydrated Sample

Mass of Crucible and Dehydrated Sample – Mass of Crucible = Mass of Dehydrated Sample

Mass of Copper

Mass of Watch Glass, Filter Paper, and Copper – Mass of Empty Watch Glass – Mass of Filter Paper = Mass of Copper

Mass of Water

Mass of Hydrated Sample – Mass of Dehydrated Sample = Mass of Water

Number of Moles of Copper

Mass of Copper from Experiment / 1 Mole Copper
Molar Mass of Copper
0.383 g Cu / 1 mol Cu
63.546 g Cu

Mass of Chlorine

Mass of Dehydrated Sample – Mass of Copper = Mass of Chlorine

Mole Ratio, Chlorine: Copper in Sample

Number of Moles of Chlorine  Number of Moles of Copper = Mole Ratio, Chlorine:Copper

0.0115 mol Cl 0.00603 mol Cu = 1.91

 2 Cl : 1 Cu

Formula of Dehydrated Sample

Mole Ratio of Chlorine : Copper

CuCl2

Formula of Hydrated Sample

Formula of Dehydrated Sample and Mole Ratio of Water : Copper

CuCl2 2H2O

Percent Error: Copper

Molar Mass of Copper  Molar Mass Copper Chloride Hydrate = Percent Composition of Copper

63.546 g Cu 170.4816 g CuCl2 2H2O = 0.37275

Mass of Hydrated Sample x Percent Composition = Theoretical Mass of Copper

0.996g Cu x 0.37275 = 0.371g Cu

Results –

I found the chemical formula of copper chloride hydrate to be CuCl2 2H2O, which is the correct chemical formula.

Error Analysis –

The percent error for copper is 3.23 percent, which is 0.012 grams too high; the percent error for chlorine it is 1.69 percent, which is 0.007 grams too low; and the percent error for water it is 1.90 percent, which is 0.004 grams too low. An explanation for this is that not all of the water had evaporated off of the copper when it was taken out of oven at the end of the experiment. The oven was opened many times, which allowed some heat to escape. Also, there were many other samples of copper in the oven that had water evaporating off. This means there was a build up of water in the oven, lengthening the time it would take the water to evaporate. We waited about an hour and a half before massing the copper, which may not have been enough time considering the build up of water and the fact that heat was escaping. It would have been assumed that the added mass of water was copper, causing the data to show the mass of copper to be higher than it should have been. This would cause the mass of chlorine to be lower than it should have been (the mass of copper was used to find the mass of chlorine). An error for the lower amount of water would have occurred at the beginning of the experiment. Possibly,all of the water may not have evaporated off of the compound when it was heated, or the compound rehydrated as it was cooling. Therefore, when the dehydrated substance was massed, there were still some water atoms in the substance. However, it was assumed that all the water had evaporated off, so any water that remained was assumed to be copper chloride. This would cause the mass of water to be lower than the actual value, which was the case. Another reason for the error is that in massing something a few times, there may have been unwanted mass. Some unwanted mass could have been oil from our hands or something such as water from the table. This would unintentionally increase the mass of that item, but only by a small value. This is a reasonable explanation, because the percent error was very low.