Density: Accuracy and Precision Using your lab Notebook You will record all lab data and calculations in your lab notebook for this course. To help you learn correct set-ups of data tables and calculations, the lab handouts in the first few labs will include example tables and suggested calculations. Since the notebook pages use “carbonless” copies, you must use a pen. Do not use a pencil since mistakes cannot be erased from the copy. Print out the “Keeping a Lab Notebook” link from the CHM151 Lab website and keep it inside the back cover of your lab notebook for reference. Typically all data and calculations will be shown only in your notebook. DO NOT fill in the data tables or calculations in this handout. You will complete the information in your lab notebook. Be sure to leave sufficient space between tables and calculations lines to show your work. It is better to overestimate than underestimate and run out of space.
Procedure 1. Before beginning the procedure, answer post lab question 1. 2. Use an analytical balance to weigh the 4 brass slugs. 3. Use calipers to measure the length and diameter of the four brass slugs. Be sure that the calipers are reading in millimeters (rather than inches). Convert the caliper readings from mm to cm. 4. Using a 100 mL graduated cylinder, place sufficient water in it to cover the sample but do not add any of the slugs yet. Place a rubber stopper on the bottom of the cylinder to serve as a cushion and prevent breakage. Measure and record the volume of the water and the stopper to the maximum precision possible as shown above. 5. Carefully insert each of the brass slugs in the graduated cylinder by tipping the cylinder and sliding the sample down the side. 6. Dry off the slugs, and return them to the container. Volume of a cylindrical solid
Volume of a rectangular solid
π · radius2 · length
1st edge · 2nd edge · length
1st edge length diameter
length nd
2 edge Figure 1
GCC CHM 151LL: Density: Accuracy and Precision
Figure 2 © GCC, 2011
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Showing Calculations All calculations must be shown in your lab notebook during the semester. To receive full credit for your calculations, the following should always be followed Write down any formulas used in your calculations. Measurements should always have units, even in calculations. When reporting calculated values (such as in a table), it should be shown with the correct number of significant figures. However, within calculations you may show extra figs. If calculations for multiple trials are required, you may need to only show on sample calculation.
Calculations 1. From the linear measurements, use the appropriate formula to calculate the volume of each slug in cm3. These calculated volumes are expressed in cm3 (cubic centimeters). The volume of a cylindrical object (Figure 1 above) is calculated by: V = π radius (cm) 2 length (cm) where r is the radius of the cylinder (half the diameter); use the button on your calculator for better accuracy. The volume of a rectangular or square object (Figure 2) is calculated by: V = length(cm) width (cm) height(cm) 2. The volume of water displaced is represented in mL and can be calculated by: (volume of water + stopper + slug) minus (volume of water + stopper) 3. Density is defined as mass/volume, and is usually expressed as g/mL (or g/cm3 in the metric system). Calculate the density for each metal slug in two ways: #1 by using the caliper measurements(units of g/cm3), and #2 by using the water displacement (units of g/mL). density
mass (in g) volume (in cm 3 or mL)
Report all values to the correct number of significant figures. Also calculate the average density value for each type of slug. 4. The accepted value of density is 8.470 g/cm3 for brass. Determine the accuracy of your average measured values by calculating the % error for each of your average density values. Percent error values are always reported as positive numbers. Note that without the absolute value bars, this can result in a negative value for the error.
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% error =
experimental value - accepted value 100 accepted value
Density: Accuracy and Precision: Lab Report DO NOT COMPLETE THE DATA OR CALCULATIONS ON PAGES 3 OR 4. THEY WILL NOT BE GRADED ON THIS HANDOUT. THEY WILL BE GRADED IN THE CARBON COPIES OF THE LAB NOTEBOOK PAGES YOU TURN IN.
In your lab notebook, create a table of mass and caliper measurements (both millimeter and centimeter) for the four brass slugs. See the following two pages for examples. DATA Density Using Calipers: Brass (4 cylindrical samples) Mass (g)
Length (mm)
Length (cm)
Diameter (mm)
Diameter (cm)
Radius (cm)
Volume (cm3)
Density (g/cm3)
Density Using Graduated Cylinder: Brass (4 cylindrical samples) Mass (g)
Volume water + stopper (mL)
Volume water + stopper + slug (mL)
Data Summary Table: Average density from calculation (g/cm3) Experimental value Brass GCC CHM 151LL: Density: Accuracy and Precision
Volume (mL)
Density (g/mL)
Average density from displacement (g/mL) Experimental value © GCC, 2011
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Significant Figures: # of decimal places in avg. calculated density
# of sig. figs. in avg. calculated density
# of decimal places in avg. displacement density
# of sig. figs. in avg. displacement density
Brass
Accuracy by percent error: Metal (Accepted Density)
Percent Error: Calculation Density
Percent Error: Displacement Density
Brass (8.470 g/cm3)
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Name: _____________________________ Group members: __________________________________
Section: ___________________
*TURN THIS PAGE IN* Post-Lab Questions: You may use the back of this sheet to answers questions, if needed. If you do, indicate that your work is on the back or your instructor may not see it! 1. Which method (circle one) do you think will give you the more accurate density in the lab today? caliper graduated cylinder Explain your choice:
2. According to your experimental results which method (circle one) of determining volume gave the more accurate density? caliper graduated cylinder
3. List one advantage and disadvantage for each measuring device (caliper and graduated cylinder).
4. List 3 ways you could change your procedure or technique to improve your results. 5. An object is known to have a density of 4.9435 g/cm3. A freshman lab class measures objects made of the same material to have the following densities: 4.57, 4.58, 4.55, 4.57, 4.55, 4.56, 4.58, 4.57, and 4.56 g/cm3. Describe the students’ data in terms of accuracy and precision and explain your answer.
6. Two objects have the same mass but different volumes. Which will be more dense, the one with the larger volume or smaller volume? ___________________ Explain your reasoning.
7. A solid cylinder of plastic has a density of 1.6 g/cm3. It is then cut exactly in half. What is the density of each of the pieces now? Explain.
8. Liquid A has a density of 0.90 g/cm3, liquid B has a density of 1.15 g/cm3, and liquid C has a density of 0.65 g/cm3. A 10-mL sample of each liquid is poured into one graduated cylinder and allowed to sit overnight. Assuming the liquids do not mix into one another, which liquid will be on the bottom, in the middle, and on the top in the graduated cylinder?
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