2—Density Name: ___________________________________________________ Date: _____________________________________________________ Section: _________________________________________________
Objectives • • • • • • •
Reinforce concepts of mass, length and volume and density Additional experience with an electronic analytical balance Additional experience with the uncertainty of physical measurements Additional experience with figures in laboratory measurements Additional experience with of average, deviation, average deviation and relative average deviation Additional experience with volumetric measurements and pipetting technique Measure the density of an unknown metal bar and measure the specific gravity of a salt solution
Pre-Laboratory Requirements • • • •
Read Chapter 1.4 in Silberberg Watch the video on using electronic balances Pre-Lab Questions (if required by your instructor) Laboratory Notebook—prepared before lab (if required by your instructor)
Safety Notes •
Eye protection must be worn at all times
Discussion The density of any substance is the ratio of its mass divided by its volume. The SI unit for density is kg/m3, and ρ (lower case Greek letter rho) is the preferred symbol for density. An italicized lower case d is also used to symbolize density and the symbol we will use. Because chemists usually work with smaller quantities than cubic meters, g/cm3 is the density unit most commonly used in chemical applications (g/cm3 is also written as g/cc or g/mL). The volume of a substance is a function of temperature. Therefore, the material’s density is also a function of temperature. The density of solids are least affected by temperature changes, the density of liquids are affected more than solids, and gas densities show the largest temperature affects. It is important to specify the temperature when recording the density of any object for this reason. Density is a characteristic property of a substance and measuring the density of an unknown object is often used to identify its composition. Iron pyrite, FeS2 or Fool’s Gold, is a naturally occurring mineral that looks like gold nuggets when found in the field. Iron pyrite can be distinguished from gold, however, with a simple density measurement. FeS2 has a density of 4.8 g/cm3, and the density of gold is 19.3 g/cm3.
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Specific gravity is another property of matter that is closely related to density. The specific gravity of a substance is the ratio of its density relative to the density of another material, and specific gravity (symbolized by SG) has no units. Specific gravity is frequently used in clinical procedures to describe the composition of solutions administered to patients during medical procedures. The specific gravity for aqueous solutions is the density of the solution divided by the density of water at the solution’s temperature. For example, if you wish to calculate the specific gravity of a saline solution (sodium chloride dissolved in water), it would be necessary to measure the density of the saline solution, and then divide this quantity by the density of distilled water at the temperature of the solution. If 10.125 g of a solution occupied a volume of 9.975 cm3 at 20 °C, the density would be: d = 10.125 g ÷ 9.975 cm3 = 1.015 g/cm3 The density of water at 20.0 °C is 0.998203 g/cm3, and the specific gravity is therefore: SG = 1.015 g/cm3 ÷ 0.998203 g/cm3 = 1.017 This experiment will provide additional experience with an electronic balance and measuring volumes. Finally, you will gain experience reporting the correct number of significant figures associated with physical measurements and working with significant figures when doing mathematical operations.
Procedure Part I-A: Mass of a Metal Bar 1. Watch the video in Canvas titled “Using the electronic analytical balance.” 2. Tare the balance so the display reads 0.000 g. 3. Open a side door to the balance and place your unknown metal bar on the balance pan and record the bar’s mass. 4. Close the side door to the balance and record the mass to the nearest milligram. 5. Remove the metal bar from the balance and verify that the display again reads 0.000 g. 6. Repeat this process two additional times, recording the metal bar’s mass for each trial. 7. Calculate the average mass of the metal bar from the three separate mass determinations.
Mass of metal bar (g)
Trial 1
Trial 2
Trial 3
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Average mass of metal bar (g)
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Average deviation of mass (g)
_______
Relative average deviation of mass
_______
Unknown Number of metal bar: _______________ Calculations:
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Part I-B: Volume of the Metal Bar with a Ruler 1. 2. 3. 4. 5. 6. 7.
Use a clear plastic ruler to measure the length of your metal bar (± 0.5 mm) and record the data. Use a clear plastic ruler to measure the diameter of your metal bar (± 0.5 mm) and record the data. Repeat these measurements 2 additional times, recording the data each time. Calculate the density of the metal bar from each set of measurements. Calculate the average density of the metal bar. Calculate the average deviation and the relative average deviation of the densities. Report the density of the metal bar for procedure I-B.
Length of metal bar (cm) Diameter of metal bar (cm) Volume of metal bar (V = πr h, cm ) 2
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Density of metal bar for Part I-B (g/cm )
Trial 1
Trial 2
Trial 3
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Average density of the metal bar (g/cm3)
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Average deviation of density, Part I-B (g/cm3)
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Relative average deviation of density, Part I-B
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Reported density for metal bar, Part I-B:
________ ± ________ or ________ ± ________ %
Calculations:
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Part I-C: Volume of the Metal Bar by Measuring Displaced Volume of a Liquid 1. Place approximately 10 mL of DI water in a 25.0 mL graduated cylinder and record the volume to the nearest 0.5 mL. 2. Drop the metal bar into the graduated cylinder and record the new volume to the nearest 0.5 mL. 3. Repeat these measurements 2 additional times, recording for each trial. 4. Calculate the density of the metal bar from each set of measurements. 5. Calculate the average density of the metal bar. 6. Calculate the average deviation and the relative average deviation of the densities. 7. Report the density of the metal bar for procedure I-C. 8. Use the known density of aluminum (2.70 g/cm3) and iron (7.874 g/cm3) to identify the composition of your metal bar and answer the questions below. (Your metal bar is composed of one of these metals.) Trial 1
Trial 2
Trial 3
Volume of water initially (mL)
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Volume of water with metal bar (mL)
________
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Volume of water displaced by metal bar (mL)
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Density of metal bar for Part I-C (g/mL)
________
________
________
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Average density of the metal bar (g/cm )
________
Average deviation of density, Part I-C (g/mL)
________
Relative average deviation of density, Part I-C
________
Reported density for metal bar, Part I-C:
________ ± ________ or ________ ± ________ %
Calculations:
Composition of metal bar: __________________________ True density of metal bar (g/cm3): __________________________ Absolute error (g/cm3): ______________________ Relative error: __________________________ Calculations:
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Part II: Density and Specific Gravity of a Salt Solution 1. 2. 3. 4. 5. 6. 7. 8.
Record the mass of a 50 mL beaker. Using a volumetric transfer pipet, pipet 25.00 mL of salt solution into the beaker and record the mass. Empty the salt solution into the sink and dry the beaker. Repeat this process 2 additional times, recording the mass for each trial. Calculate the density of the salt solution for each trial. Calculate the average deviation and the relative average deviation of the salt solution. Record the temperature of the salt solution (±0.1 °C). Use the density versus temperature table (posted in the laboratory) to find the density for water at the temperature recorded. 9. Use the density of water from step 8 to calculate the specific gravity of your salt solution. 10. Report both the density and the specific gravity of your salt solution. Trial 1
Trial 2
Trial 3
Mass of empty 50 mL beaker (g)
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Mass of beaker with 25.00 mL salt solution (g)
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Mass of salt solution (g)
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Volume of salt solution (mL)
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Density of salt solution (g/mL)
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Average deviation of salt solution density
________
Relative average deviation of salt solution density
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Reported density for salt solution:
________ ± ________ or ________ ± ________ %
Temperature of salt solution: _______________ °C Density of water at this temperature: ____________________ Specific Gravity of Salt Solution: __________________________ Calculations:
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Final Report Your final report should compare the uncertainty of the density measurement obtained from method I with the uncertainty of the density obtained from method II. Answer the questions “Which method gives more precise answers?” and “Which method gives more accurate values for the density?” Is the specific gravity of your salt solution greater than the density of the salt solution, equal to the density of the salt solution or less than the density of the salt solution? What change would you predict in the value for specific gravity if your salt solution were warmed to human body temperature (37 °C)? Explain the answers to these questions in the conclusion of your report. Be sure to use the proper number of significant figures in your written report, both in the tables and in the text.
Reference American Chemical Society, What is Density? Middle School Chemistry, Washington, DC, 2011; pp 140-149.
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