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Accuracy vs. Precision Accuracy vs. Precision •AccuracyAccuracy -How close a measured value (average value) is to the actual (true) value. - % error from actual value.
•PrecisionPrecision -How close the measured values are to each other. - % difference from average value.
Low AccuracyHigh Precision
High AccuracyLow Precision
High AccuracyHigh Precision
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% error = -------------------------------- X 100 %Experimental value - Actual valueActual value
Fall 2009 Quiz #2 (in class)Fall 2009 Quiz #2 (in class)50 ml of H2O at 2l oC (true Density = 0.998 g/ml ) is pour into a beaker of mass 55.23g. The mass of the beaker & water is 63.39g. Calculate the density of H2O, Calculate the % error. Comment on accuracy and precision.
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2. (6 points) You are asked to review the following mass-volume data:Table 1. Mass and Volume of Water at 25oCYou see that the experimenter graphed volume of water on the y-axis and mass of water on the x-axis using the Graphical Analysis software. A linear fit of the data points gives a slope of 1.02 and a correlation of 0.998. You use Graphical Analysis to determine the density of water from the three data points to be 0.962 g/ml.
Spring 2005 Quiz 1Spring 2005 Quiz 1
Run Mass of Water, g Volume of Water, ml1 9.87 10.02 20.55 20.03 29.11 30.0
a. Based on the volume of water in Table 1, which volume measuring device did the experimenter use: a 10 ml graduated cylinder (uncertainty = 0.05 ml), a 100 ml graduated cylinder (uncertainty = 0.5 ml), or a 100 ml beaker (uncertainty = 5 ml)? Give reasons.b. Explain why you got a different density than the experimenter. Who got the correct density? Give reasons.
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3. (6 points) The following is a label taken from a 1.5 liter (50.7 oz.) bottle of Castle Rock Spring Water.Nutrition Facts: Serving Size 8 oz. (240 ml) ANALYSIS (mg/liter)About 6 servings per container Calcium .......................... 10Amount per serving Magnesium ...................... 5Calories 0 Bicarbonates .................. 65Total Fat 0 g Potassium ........................ 2Sodium 0 mg Chloride ........................... 2Total Carb. 0 g Nitrate ........................... <1Protein 0 g TDS Approximately 80
(TDS stands for Total Dissolved Solids)a. Compare the analysis of calcium to bicarbonates on the label. Which substance has the larger uncertainty in the reported value? Provide reasoning to support your answer.b. The label states that the bottle contains 1.5 L of water. How many grams of magnesium are present in this bottle of water?c. The six substances (ignore TDS) in the Analysis section on the label exist in water as ions. You boil off all of the water and see a solid at the bottom of the container. Give the name and chemical formula of one compound that this solid could be. Identify this compound as molecular or ionic.
Fall 2001 Quiz 1 & Spring 2005 Quiz 1Fall 2001 Quiz 1 & Spring 2005 Quiz 1
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You and your lab partner are measuring the properties of a colorless liquid. The following data were taken with a single-pan analytical balance for mass and a graduated cylinder with graduations every 1 ml for volume:
Run mass of liquid, g volume of liquid, ml1 4.967 5.002 10.002 10.003 14.896 15.004 19.978 20.00
a. Are the significant figures in the volume data reported correctly? Give reasons.b. Using Graphical Analysis, plot a graph of mass of liquid (y-axis) vs. volume of liquid (x-axis). c. What property does the slope of the line on this graph represent?d. What is the identity of this liquid?e. The true value for this property is 0.988 (units). What are the units of this property?f. From the true value from part (d), does the above data show a random error or systematic error? Give reasons.
You are measuring the volume of a solid by two methods: (1) ruler measurement of length, width, and height, and (2) liquid displacement. You obtain the following data:
Calculations are not required to answer these questions. (1 cm3 = 1 ml)a. Based on the above data, which method shows the greater precision? Give reasons to support your answer.b. Based on the above data, which method shows the greater accuracy? Give reasons.c. Based on the volume by ruler data, do you think the solid has a regular shape, e.g., a cube, or an irregular shape? Give reasons.d. Based on the volume by liquid displacement data, what are the graduations on the graduated cylinder, e.g., 10 ml, 5 ml, 1 ml, 0.1 ml, etc.? Give reasons.
3. (10 points) You find a blue rock and a green rock in your backyard. Each rock is approximately the same mass. The blue rock has a larger volume than the green rock.a. Which rock is more dense? Give reasons.b. You take each rock and break them into different sized pieces. For each piece, you measure the mass and volume. You graph the mass of each rock (y-axis) vs. the volume of each rock (x-axis). On the graph below, identify which line represents the blue rock and which line represents the green rock. Give reasons for your choice.
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mass, g |||||_________________________
volume, mlc. Describe how you could use the results of your experiment to identify the type of rock.
3. (13 points) An interviewer for your first job as a lab technician asks you to describe how you would calculate the density of a colorless liquid from the following data:
Run Volume, ml Mass, g Density, g/ml
1 10.00 7.9452
2 20.00 16.0025
3 30.00 23.8276
4 40.00 36.2584
5 50.00 39.5899
Average Density =
a. Based on the significant figures shown in the Volume column, which volume measuring device do you think was used? Give reasons.b. For each run, calculate the density of the liquid. Then, calculate the average density. (Extra credit: 1 point. Use an Excel spreadsheet to calculate density. Print your spreadsheet and turn it in.) Comment on the precision of these 5 runs. (See the Chem 1A Lab Manual, p. 12 and Moore, “The Chemical World”, p. A-6 for precision.)c. Using the Graphical Analysis software, plot mass vs. volume. Use the mouse to draw a box around all of the data points on the graph. Then, go to the Analyze pull down menu and select Regression. Record the slope (M), intercept (B), and correlation coefficient (COR). Print out your graph and turn it in. Calculate the density of the liquid from your graph. Comment on the precision of these data. d. Compare the data in the table and in your graph. Are all the data points “good” or should one or more data points be discarded? If a data point should be discarded, identify the data point and explain why this data point should be discarded.e. Compare the average density you calculated in part b to the density from the graph in part c. Which density do you “trust”? Give reasons.f. The colorless liquid is either water, ethanol, or hexane. Look up the density of each substance at 25oC. Cite the reference where you found this information. Based on the density data, what is the identity of the liquid? Calculate a % error. Show your calculations.