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JAMES 0. SCHRECK UNIVERS~TY OF NORTHERNCOLORAW CSIEELEY. CO SO639 -. .-
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Determining theElative Masses of Copper and lron Atoms Bernal A. Kiser, Jr. Notre Dame de la Baie Academy, Green Bay, WI 54303
A critical term in quantitntivechemistry is the term"relative". For examole. the oeriodic table indicates the relative masses of the a&& of ihe elements, yet it does not record the mass of any single atom. The purpose of this laboratory activity is to help the first-year chemistry student unravel this dilemma. Specifically, the student will determine the relative masses of copper and iron atoms. This determination is based on the premise that the numbers of iron atoms "invested" in the chemical reaction and the number of copper atoms produced are in "equal billions". The equation for the reaction studied is:
Procedwe 1. Record the mass of about 1.5 g of the "tuft" of steel wool (to the nearest centigram) on a piece of weighing paper. 2. Similarly, record the weight of approximately 17 g of blue, crystalline copper sulfate (CuS04.5Hz0) on a piece of weighing paper. 3. By means of a "paper funnel", transfer the blue crystals into a 250-mL Erlenmeyer flask containing 200 mL distilled water. Swirl to effect solution. If necessary, heat gently to completely dissolve the crystals. 4. When the majority of the solid has dissolved, place the "tuft" of steel wool into Erlenmeyer flask. Use a stir rod to poke the tuft beneath the surface of the blue solution. Do not stir. Record observations in your laboratory notebook. 5. After the "tuft" has dissolved, proceed as follows: (a) Determine the mass of a piece of filter paper. (b) Fold the filter paper to fit a short stem funnel, and place it in the funnel. Wet the filter with distilled water. (c) Filter the contents of the Erlenmeyer flask onto the oreweiehed filter . oaoer. . Catch the filtrate in a 250beiker. id) . . When the filtration is comolete. discard the filtrate. and retain the copper-con&ining filter cone for dry: ing and weighing. (e) Place the filter cone and copper residue within a labeled 100-mL beaker. Set the beaker in a drying oven, or let it set undisturbed in a suitable place for drying. 6. At the next laboratory period, record the mass of the dry filter paper and copper-containing residue. Discard paper and residue in the waste can.
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Calculations 1. Assuming that for each atom of iron reacted, one atom of copper is formed, determine the relative masses of copper and iron atoms, using the formula: R = ?a-s X billions .Cu . atoms = mass individ@&aarom maw individual Fe atom meal msss X bullions Ye sroms
2. Calculate the "periodic tahle ratio" of the two atoms.
250
Journal of Chemical Education
3. Calculate the (a) error difference, (b) percentage error, and (c) percentage accuracy for your experimental result. Sample Data A typical set of data and appropriate calculations for this experiment follows: 1.58 g Mass of steel wool reacted 16.88 g Mass of copper sulfate Mass filter paper 1.00 g 2.79 g Mass filter paper + copper residue Step-by-step Calculations 1. The msss of iron reacted is the quantity of steel wool used-1.58 g.
Differentiatehetween a direct versus a "derived" quantity. 2. The mass of copper metal recovered is determined as follows: maas filter paper + copper residue 2.19 g mass filter paper 1.00 g mass copper residue 1.79 g 3. Determine the experimental value for Rc,pp R = Cu atomsme atoms = 1.79 gl1.58 g = 1.1311.00 4. Determine the class average R c , p . Typical results fallow: M a s Copper (grams) Mass Iron (grams1 1.58 2.03 2.09 2.15 2.00
2.36 1.61 2.38
1.79 2.46 2.39 2.50 2.28 2.70 1.80 2.71 2.24
18.20 20.91 R = Cu atomsme atoms = 20.91 g118.20 g = 1.1511.00 5. Determine the theoretical value of Cu/Fe using the periodic tahle of the elements RCllpe= 63.546 amu155.847 amu = 1.1411.00 6. Calculate: (a) error difference,(b) percentage error, and (c) pereentageaccuracy. a. Error difference (cla9svalue)- theoreticalvalue = 1.15 1.14 = 0.01 b. Percentage error = 0.0111.14 X 100%= 0.88% e. Percentage accuracy = 100.00%- 0.88%= 99.12% 7. Ask the students to conclude what the experimental results indicate about the relatiue masses of a copper atom to an iron atom? Preferred response: "The findings indicate e single mpper atom to have a greater mass than a single iron atom by afactor of 1.15."
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Summary This experiment represents an activity using familiar and simple materials, giving the student a first-hand understanding of the terms "relative mass" and "by a factor of'. General Reference MCQ,,,~~, h n s l d A,; Rock Petor A. General Ch~mirfry.2nd Yo&, 1987:Chspter 1.
od.: W.H.Freeman: New
