Sample Data Mass of solid stopper Volume of water displaced by solid stopper Volume of water displaced by two-holed stopper Volume of rubber removed from two-holed stopper (15.0mL-11.8 mL=3.2 mL) Density of rubber in stopper (16.62 gl15.0 mL = 1.I g/mL) Mass of rubber removed from two-holed stopper (3.2 mLx 1.1 g/mL = 3.5 g)
16.62g 15.0 mL 11.8 mL 3.2 mL
identical neither to each other nor to the solid stoppers before the holes were drilled. Due to this, grading the lab report on the accuracy of the results is not recommended. However, the written procedure used to obtain the results can be evaluated. Conclusion The procedure given above is a somewhat crude way to accom~lishthe stated obiective. The fun art of this labor a t o j i s obsening the &dents prepare apply the density concept to a new situation. This laboratory exercise gives students an opportunity to use higher-level thinking skills in developing a plan to solve a problem.
to
3.5 g
poured into a 25-mL graduated cylinder. The volume of water measured is the volume of the solid stopper. 4. The students can now calculate the density of the solid stopper using the following formula: mass density = volume
5. The students then repeat step 3 with the two-holed stopper and obtain its volume. (Allow the air bubbles attached to the holed stopper to dissipate before recording the volume.) Students can also assume that the holes in the stopper are shaped like a cylinder, calculating the volume of the holes with the following formula: Volume = 2nR2h where R is the radius of one of the holes; and h is the height of the hole. When rulers are used, this mathematical method is not very precise. 6. Subtracting the volume of the two-holed stopper from the volume of the solid stopper, the students then calculate the volume of rubber removed from the two-holed stopper. 7. Finally, the mass of rubber removed from the twoholed stopper can be calculated (see the table): mass of rubber = volume of rubber x density of mbber
Literature Cited 1. Tzimopoulos, N.D.;Metealfe, H. c.;Williams. J. E.:CastIra, J. F.M&m
Erratum: Chromatography of M&MCandies
,I
The last Filtrates and Residues feature, which ap-
3:
the paper by Marjorie Kandel, "Chromatography of M&M Candies" (page 98S989). The table as it should , : have appeared is reproduced below.
!: peared in the December 1992, omitted a table from ;
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Colors and Rt Values of the Candy Shells and Dyes Sample
16.62 g- 14.60 g = 2.02 g
Journal of Chemical Education
Colors after development
;j
FA valuesa
Yellow candy Orange candy Red candy Green candy Brown candy
wt of the solid stopper - wt of the two-holed stopper =
150
:14
1,
Discussion The results from this data were checked by comparing the experimental result (3.6 g) with the actual difference between the masses of the two stoppers. This is obtained by weighing both stoppers and then subtracting their masses:
The high percentage error obtained (80%)may be due to the crude procedures used to obtain the volume or the inaccurate assumption that the stoppers were identical before the holes were drilled. Several two-holed stoppers were weighed, and masses of 13.87 g to 14.97 g were obtained. This indicates that the two-holed stoppers were
Chemistry
LzbrotoryExpp?imnt.: Holt.Rioehartaod Wmton:Austin, 199(1:Erperiment3, p 30. 2. Davh, J. E.. Jr;Msnab. W.K.; McCIe1la0,A L.; O'Crmncrmn, P. R.Lzbormo'y M=nuol lor C&mis~:E%ptimnUondPh'mipl~s; D.C. Heath: Icdngtoh MA, 1982;E= penment 4, p 4. 3. Camhehael,L.N.;Haines, D.F;Smoot, R.C.Lzbomt~ryChemiahy;Merrill: Columbus, OH, 1983; b l l m e n t 3, p 25. 4. Bolton, R. P;lemphere. E.Y: Menesine, M.Action Ckrniafry;Holt. Rinehart and Winston: New York, 1979: p 20.
1 1
Yellow 0.43 Orange 0.35 Red 0.21 0.68,0.41 Blue, yellow 0.67.0.46, 0.37?, Blue, yellow, orange?, red 0.16 Tan candy Blue, yellow, 0.67.0.48, 0.30?, orange?, red 0.18 Tartrazine Yellow 0.54 Food color Yellow 0.58 'Except tor the lwd coor, whcn was measurm trom as ngle n n on CnromalaoraDhv mwr. Rt "alms were determsnm tram averaOlno2-3 wns, one'bnkilter'arid1-2 onchromatography paper. Tailing ma& iihard to determine exact spot positions, but the color composition (with the
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