Chemical Education Journal (CEJ), Vol. 7, No. 2 /Registration No. 7-15/Received October 10, 2003.
URL = http://www.juen.ac.jp/scien/cssj/cejrnlE.html

mail: gruvberg@kreativkemi.se

Presentation
Experiment 1
Experiment 2
Summary

Presentation:
Two experiments: "The Number of Water Molecules in a Salt" and "Colorimetric Determination of a Copper Sulfate Solution" used in sequence make a combination of great value for the student's understanding of stoichiometrics. Both experiments are using cheap and simple equipment.
The first experiment makes the student aware of the fact that the blue color in the hydrated copper sulfate is dependent on both the copper ions and the water; when water is evaporated from the salt the color is vanishing as well; when water is added to the anhydrous salt the blue color reappear.
The calculations on the ratio of water molecules per salt unit are making the student familiar with the stoichiometric ideas. As a bonus the manipulations demonstrate that heating breaks the bonds to the water molecules and that heat is released when the salt is hydrated again.
The second experiment demands logics combined with the recent acquired experiences as well as a retrospect on the theory picked up during the lectures. The later experiment contains a discovery experience when the students find out that the color intensity (seen along the length of the tube it is in, see the arrows in the tubes in the figure below) is independent of diluting. The observed result is surprising and it is challenging the students to understand why. When the students are able to explain the phenomena as "same color means same number of moles" (n₁=n₂) then they are also able to convert the formula into c₁V₁=c₂V₂, and because the tubes are identical the bottom areas are equal which simplifies the formula to c₁h₁=c₂h₂ ( n is number of moles, c is the mole concentration, V is the volume and h is the distance through the liquid when the color intensity was visually estimated). The instruction for preparing the analyzed solution is given in g/L, the colorimetric result is given in moles/L. When the grams per liter are converted into moles/L the students see the relation between mass and moles. Beside a lot of cognitive work they also gain a lot of manipulative skills useful in most common laboratories.

Experiment 1: "The Number of Water Molecules in a Salt".
Instrumentation: Cut off Pasteur pipettes, Blue fresh hydrated copper sulfate salt (CuSO4 5H20), spatula, tripod, wire gauze, micro-torch and mg-balance.

Procedure:
Cut off the Pasteur pipette at the spot marked with the broken line. Weigh the empty pipette to the nearest mg on the balance. Spread out some 0.3g blue salt. Weigh the pipette with salt to the nearest mg. The difference between the two mass values is the mass of blue salt. Place the wire netting on the tripod and place the pipette on the netting. Heat the pipette gently using the micro torch or any available burner. It is important not to heat too hard in order to avoid sulfur dioxide release from decomposing sulfate. It is also important to heat the whole pipette to stop the escaping water molecules from condensing in the pipette openings which can cause splattering and loss of salt.
Continue heating for another minute when the salt has lost its blue color. Weigh the pipette when it has cooled for a few minutes. The mass difference between the two latest weighings is the mass of lost water. Estimate the ratio between the number of water molecules and copper ions after the masses has been calculated into moles. The expected number is five, in practice the value is often closer to 5.2.

Experiment 2: "Colorimetric Determination of a Copper Sulfate Solution".

Comparation (comparing the color intensities seen from the side)

Colorimetrics (comparing the color intensities seen from above)

Instrumentation: Two 12mL plastic colorimetric tubes, ruler, reference solution (0.10M CuSO4), filter paper, 10 mL volumetric flask, weighing boat, mg-balance.

Procedure:
Prepare a solution with 15.0 g blue salt per liter in the 10 mL volumetric flask. To speed up the dissolving of the salt, the solution can be warmed up by pouring warm tap water on the outside of the flask.
Fill the two colorimetric tubes to one third of its volume (the same volume in both tubes) with a colored solution (red food coloring is recommended). Predict what the intensity of the colors in the tubes will be, seen from above, (as pointed out in the first figure) when compared to each others. Compare the colors by looking through the solutions from above with a white filter paper, illuminated with good light, as the back ground. Like everyone expects the intensity is the same in the two tubes. Now, predict what impact on the intensity diluting one of the solutions, with water to the double volume, will have. Most students predict that the solution will be brighter, a few that it will be darker and very few that the dilution will not affect the intensity of the color. The correct prediction is ''No change'', this is obvious when considered that the number of moles for the molecules that generates the color is the same after dilution. When observed along the length of the tube the observation passes the same number of coloring particles no matter how large or small the volume is. The spontaneous prediction is based on the experiences from observations made from the side when a colored solution (for instance a fruit syrup)is being diluted. As mentioned earlier in the text from this point the students discuss how this result can be used for generating a formula and a method for determining the mole concentration of the solution they have prepared by using the reference solution with the known concentration 0.10 M. They usually generate the equation from which they can work out a method without any problems. To construct a simple and effective method using their observations and their equation takes them a little more of intellectual work. The most common way to determine the concentration that the students work out is to pour the 10 mL solution they have prepared into one tube and then they fill the other tube with a Beral pipette until they have the same intensity in color in the two tubes. They measure the heights (h1 and h2) of the solutions and after using the values to solve the equation they have a result that is within 10 % error from the correct value. Finally they convert the instructed mass concentration (15.0 g/L) into moles per liter and find that the two results are comparable.

Summary:
Two experiments in sequence use the same salt for quantitative measurements, one is using a gravimetric method and one is using a colorimetric method. One is determining the number of crystal water molecules in copper sulfate, one is determining the mole concentration of the same salt in a solution that they have prepared with high accuracy. The two experiments give use to a lot of the theory in stoichiometrics that they have learnt during lectures and in problem solving. The use of the same salt in both experiments improves the value of the second experiment as this offers them a situation where they can draw conclusions built on experiences that they picked up in the first experiment. In order to get good results they have practiced methods they can use in other occasions, they have been trained to work with accuracy combined with awareness of the benefit to do so. My experience is that the students are engaged in these two experiments and they give feed back that this sequence is effective in bringing understanding to their studies.

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