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Experiment 2: Precision and Accuracy of Measurements
II. Data and ResultsA.Balance Use: When is a gram, a gram?Table 1. Mass of five packets of sweetener weighed on one balance, % error from true value Step A-2Packet #Mass Packet + Contents(g)Mass Contents, Calculated (g)% error Contents11.26421.27231.23941.24051.190AverageStandard DeviationTable 2. Mass of one packet of sweetener weighed on five balances; % error from true valueStep A-3Balance #Mass Packet + Contents(g)Mass Contents, Calculated(g)% error Contents31.25841.25751.25561.25721.259AverageStandard DeviationB.Measuring LiquidsMass Sweetner, Nominal(g)1.0gMass Empty Packet(g).211g Balance # , Used in step A-2True Mass =Nominal Mass =1.0g
Table 3. Precision of 25 mL, 100 mL, and 1L Graduated Cylinders Cylinder Size (mL)Smallest Graduation (mL)Estimated Precision (±ml)25.2 mL(+/-) .1 mL100.2 mL(+/-) .1 mL100010 mL(+/-) 1 mLB. 2. Delivering liquid with various glassware C.Serial Dilutions: Making Solutions with Known ConcentrationsKool-Aid Color: BlueMass of Kool-Aid Used (g):.101g Table 5. Mass of 10 mL of Water Deliverd by a 10 mL Serological PipetVol. water(ml)Mass Beaker + Water(g) Mass Water, per 10 ml aliquot(g)Vol. calculated, per 10 ml(ml)063.0711072.9182082.7433092.666AverageStandard DeviationTable 6. Mass of 10 ml of Water Delivered by a 10 mL Volumetric Pipet Vol. water(ml)Mass Beaker + Water(g) Mass Water, per 10 ml aliquot(g)Vol. calculated, per 10 ml(ml)063.0711073.0612083.0443093.033AverageStandard Deviation
Table 7. Absorbance Measurements for Diluted Kool-Aid Solutions Test TubeVolume of Kool-Aid StockVolume WaterAbsorbance114.108223.201332.295441.410550.491Sample Calculations 1.Concentration of Koolaid solution made in 100 mL volumetric flask 2.1 x 10-3M = x mols / .1 Lmols= (1 x 10-4M) x (1 x 104 g)/(1 mol) = 1 g1.006 g x (1n /1x104) = 1.006x10-4/.10000L = 1.006x10-3M3.Concentration of solution in test tube 1: 1122C VC V=10 mL x 1.006x10-3M=C2x 100mLC2= 1.006 x 10-4 MKool-AidColor Transmitted_________________ Color Absorbed __________________ λmax, absorbed(nm) _______
III. DISCUSSION Part A. DiscussionThe manufacturer achieved precision and accuracy in putting 1.0 grams of sugar in each sugar packet, because the mass of sugar within each sugar packet was within the range of .999-1.060, averaging to 1.0238, and having a standard deviation of .025352. The manufacturer achieved accuracy in obtaining 1.0 gram of sugar in each packet for the most part, with the mass of sugar in each packet having percent errors of 3.1% or below and staying with in ±.5 of 1.0 grams. For only one mass was the percent error 6%, which was not accurate to the manufacturer’s description of 1.0 grams. If the manufacturer had stated each packet contained 1.000 grams of sugar then the measured masses of sugar would still be precise, because the standard deviation is .025352, or approximately ±.03 of the mean mass, 1.0238 grams. However, most of the measurements (1.029,1.031, and 1.060 grams) would not be accurate because they would deviate out of ±.001 of 1.000 grams.