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EXPERIMENT: DENSITY (non-computer version) SUBMIT THIS ENTIRE PACKET FOR GRADING; INCLUDE YOUR GRAPH.

NAME:

PARTNER'S NAME: SECTION:

OBJECTIVE The densities of a wooden block, an unknown metal, water, and a series of salt water solutions will be determined. Percent error is calculated for some of these density measurements. Graphing techniques will also be explored. DISCUSSION The density of a substance (D) is defined as the ratio of its mass (m) to its volume (V). Density = m / V For a solid, the units of density are expressed in grams per cubic centimeter; for liquids, grams per milliliter are used. The densities of regular solids such as cubes or cylinders can be determined by direct measurement and applying geometric equations. For example, the volume of a block equals length X width X height. Densities of irregularly shaped solids must be determined by indirect methods, such as water displacement. Note that density changes with temperature, so it is sometimes necessary to report the temperature at which density is determined, especially for liquids. In this experiment, densities you determine will be compared to accepted values. The formula for calculating Percent Error is shown here: Experimental Value - Actual Value % Error = _____________________________________ X 100 % Actual Value Note the use of "absolute value" brackets. The "actual" value for densities in today's lab appear in the Tables labeled "Metals" or "Water". PROCEDURE I. Density of Solids A. Density of a Wooden Block 1. Record all data on the data sheet. Obtain a solid wooden block from the cart and use a metric ruler to measure its length, width, and height. Make all measurements in centimeters and record the measurements to the proper number of digits. (Remember- report one "slot" better than the smallest mark on the measuring tool.)

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2. Calculate the volume of the block using the proper mathematical formula previously discussed. DO NOT ROUND AT ANY POINT IN THE LAB TODAY UNTIL YOU ARE ASKED FOR “DENSITY.”

3. Weigh the block of wood. Determine its density and record it on the data sheet. B. Density of Unknown Metal 1. Obtain an unknown sample of metal from the cart and record it's code on the data sheet. The code is stamped onto the metal. Make sure the metal is dry before you weigh it. 2. Obtain a 100 mL graduated cylinder from your drawer and add about 50 mL of tap water. Read the bottom of the meniscus and properly report the actual volume of water in the cylinder. Remember the rules for reporting the proper number of digits. 3. Tilt the cylinder and carefully slide the metal into the water. Return the cylinder to the upright position and record the new volume you now see in the cylinder. The change in volume represents the volume of the metal. This is referred to as "volume by displacement." 4. Calculate the density of the metal sample. Compare the density of the metal with the actual densities reported in the "Metals" Table. Identify your unknown metal by choosing the closest density in the table. 5. Calculate your percent error. Beware of the “mixed operations” calculaton. II. Density of Liquids A. Density of Water 1. Place about 40 – 50 mL of deionized water into an Erlenmeyer flask. 2. Obtain a small beaker and record its mass on the data sheet. Carefully pipet 10.0 mL of the deionized water into this beaker. Ask your instructor if you have trouble with pipetting. The pipet you will use is known to be accurate to 10.0 mL. 3. Weigh the beaker and water. Determine the mass of the water by the difference of the two weighings. 4. Measure the temperature of the water. Make sure the thermometer BULB is totally submerged to get the best reading. Record the temperature properly. 5. Look at the "Water" Table and choose the temperature that is closest to the

temperature you measured. Use the "actual" density value shown in the table to

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calculate percent error for your calculated value. Record your percent error. Beware of “mixed operations.”

B. Density of Salt Water "Unknown" 1. Obtain a clean, DRY 20 or 25 mL graduated cylinder. If you do not have one in your drawer, look on the cart. Record the mass of the cylinder. You should probably also record the "Balance #" you used and WILL use throughout this part of the procedure. 2. Obtain one sample of an "UNKNOWN" and immediately record the code number

(it will be either "A", "B", "C", or "D".) Add approximately 15 mL of the unknown into the cylinder and properly record the volume. Note that if the cylinder has any type of marks in the “tenths”, you should report to the hundredth.

3. Weigh the cylinder that now contains the unknown solution. Determine the mass of the liquid by subtracting your two weighings. 4. Calculate the density of the unknown. C. Density of Salt Water "Knowns" 1. Next, repeat the procedure from part B for each of the "KNOWN" samples. These are labeled 4.0%, 8.0%, 12.0% and 16.0%. These samples may be run in any sequence, but you should see a gradual increase in density as the salt concentration increases. You need not dry the cylinder each time. Rather, rinse the cylinder with the sample you are about to measure, dump that rinse down the drain, then refill to approximately 15 mL. Then proceed to take your volume and mass measurements. Record all data in the blank table that appears on the data sheet. 2. Once your data is complete AND IT LOOKS REASONABLE, you are to prepare

a hand-drawn graph using paper located in your lab manual after page 77. Follow the instruction sheet issued by the instructor. You will graph "Concentration" versus "Density" from the table you just completed. Then use your graph's standard curve to predict the concentration of your unknown sample you measured in Part B. You will report this answer on your handout.

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"METALS" DENSITIES

METAL DENSITY (g/cm3)

METAL DENSITY ( g/cm3)

magnesium (Mg) 1.7 cadmium (Cd) 8.7 aluminum (Al) 2.7 copper (Cu) 9.0 zinc (Zn) 7.1 "wood" metal 9.8 tin (Sn) 7.3 silver (Ag) 10.5 iron (Fe) 7.8 lead (Pb) 11.4

"WATER" DENSITIES

TEMP (° C)

DENSITY (g/mL)

TEMP (° C)

DENSITY (g/mL)

20 0.99823 28 0.99626 21 0.99802 29 0.99598 22 0.99780 30 0.99568 23 0.99757 31 0.99537 24 0.99733 32 0.99506 25 0.99708 33 0.99473 26 0.99681 34 0.99440 27 0.99654 35 0.99406

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DATA SHEET: DENSITY LAB I. Density of Solids A. Density of Wooden Block 1. Mass of wooden block _________________________ g 2. Length of wooden block _________________________ cm 3. Width of wooden block _________________________ cm 4. Height of wooden block _________________________ cm 5. Calculated volume of block _________________________ cm3 6. Density of wooden block _________________________ g/cm3 •Based on your calculations, would you expect this block of wood to float in room temperature water? Answer YES or NO and explain WHY: B. Density of Unknown Metal Unknown Code: __________ 1. Mass of metal sample _________________________ g 2. Water level before immersion of metal _________________________ mL 3. Water level after immersion of metal _________________________ mL 4. Volume of metal sample _________________________ mL 5. Calculated density of metal sample _______________________ g/mL 6. Identity of unknown metal from "Metals" Table _________________________ 7. Percent Error _________________________

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II. Density of a Liquid A. Density of Water 1. Mass of beaker _________________________ g 2. Mass of beaker and water _________________________ g 3. Mass of water _________________________ g 4. Volume of water delivered by pipet 10.0 mL 5. Calculated density of water ________________________ g/ mL 6. Temperature of water _________________________ °C 7. Density of water from Table _________________________ g/mL 8. Percent Error _________________________ B. Density of Salt Water "Unknown" Unknown Code: ___________ 1. Mass of DRY graduated cylinder _________________________ g 2. Mass of graduated cylinder and solution _________________________ g 3. Mass of solution _________________________ g 4. Volume of solution _________________________ mL 5. Density of solution _________________________ g/mL

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C. Density of Salt Water "Knowns" 1. Mass of DRY graduated cylinder from Part B _______________________ g

SAMPLE 4.0% 8.0% 12.0% 16.0% Unit Mass of cylinder and solution

Mass of solution

Volume of solution

Density

You will use graph paper to graph "Concentration" versus "Density" from the table you just completed above. Then use your graph's standard curve to predict the concentration of your unknown sample you measured in Part B. You will report this answer on your handout. Note: Assigned problems appear on page 8.

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ASSIGNED PROBLEMS: SHOW WORK AND UNITS FOR ALL PROBLEMS 1. Mercury has a density of 13.53 g/mL. What is the mass of 345 mL of mercury? ANSWER: ___________________ 2. How much water will be displaced from a full beaker of water when 0.987 g of silver is placed in it ? (See "Metals" Density Table) ANSWER: ___________________ ************************************************************************ Please note that when units are cubed, the conversion factor differences get cubed as well. For example, even though there are 100 cm in 1 m, there are NOT 100 cm3 in 1 m3. Rather, there are (100)3 cm3 in 1 m3. FOR THE NEXT TWO PROBLEMS, YOU MAY WISH TO CONVERT THE LENGTH MEASUREMENTS TO CENTIMETER UNITS BEFORE CUBING THEM TO AVOID THIS ISSUE. 3. A block of lead measures 3.5 meters by 6.0 meters by 2.0 meters. What is the mass of the block in kilograms ? (See "Metals" Density Table) ANSWER: ___________________ 4. A 3.55 ounce metal block has dimensions of 0.45 in X 2.5 in X 3.0 in. What is the density of the metal in g / cm3. See your text for help with these conversion factors. ANSWER: ___________________

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GRAPHING INSTRUCTIONS You performed an experiment where the density was determined for NaCL water solutions of known concentration. A NaCL water solution of unknown concentration was obtained and its density was determined also. You are to properly graph the four data points determined for the known solutions, then use the graph to determine the concentration of the unknown solution. THE GRAPH • graph paper is available at the end of this printed lab

• graph may be completed during lab or at home

The following rules must be learned and followed:

• use pencil or erasable ink

• data must cover as much of paper as possible (important!)

• need not start graph axes at zero

• circle data points

• draw best straight line; it need not touch all data points

• upon completion of your graph, use graph line to predict the concentration of the unknown salt solution; unknown should not appear as a data point, use dashed line to place it on graph (see graph example) • graph must have title

dimension labels (example: "length")

unit labels (example: "cm")

SEE NEXT PAGE FOR AN EXAMPLE OF A TYPICAL GRAPH After completing the graph of our data, record your answer below and include this sheet with your lab data sheets. Submit your graph. THE CONCENTRATION OF YOUR UNKNOWN IS __________

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