Che-1A: DENSITY AND LAB TECHNIQUES

OBJECTIVES:

· Use the water displacement method to determine the density of an unknown metal.

· Determine the density of distilled water and seawater.

· Apply and differentiate between precision and accuracy using your data from the experiment.

· Learn how to use basic lab equipment correctly (graduated cylinders, graduated pipets, volumetric pipets and balances) as well as the technique of pipetting.

INTRODUCTION:

Density is a physical property which helps identify particular substances. Density is defined as

D = mass Equation 1

volume

Typical units of density for liquids and solids are g/mL or g/cm3 where 1 mL = 1 cm3 (an exact relationship). Density is an intensive property which means it does not depend on the amount of material. For example, if you were to compare the density of a pure aluminum can with the density of a pure aluminum bicycle frame, the densities would be the same. The masses may be different, however, but the RATIO of mass to volume is identical since the material it is composed of is the same. Density is dependent on the physical state of matter (solid, liquid, or gas) as well as the temperature of the matter. For example, the density of liquid water is more than the density of ice. This difference in density of water allows for ice to float on top of water which protects the aquatic life beneath. The density of liquid water (the same physical state) also changes with temperature. Therefore, when you look up densities of different materials, the temperature is provided. In this experiment, you will be using basic lab techniques to determine the density of distilled water, the density of a saline solution comparable to seawater, and the density of an unknown metal. You will then process your data using Excel and then use precision and accuracy to describe the quality of your experiment.

LAB TECHNIQUES

You will learn how to use a balance, graduated cylinder, volumetric pipet (or pipette), and graduated pipets correctly in this experiment as well as the technique of pipetting. There are many types of pipets that are available (see Figure 1). This experiment uses a 10-mL volumetric pipet (see Figure 1) and a 10-mL graduated pipet. Volumetric pipets deliver a specific volume based on the volume of the pipet. The volume delivered by a pipet is typically reported to +0.01 mL. Therefore, you would report 10.00 mL of solution transferred using a volumetric pipet. Your lab drawer contains a 10-mL volumetric pipet and you will be using this pipet for the experiment.

There are several types of graduated pipets (Mohr, serological, etc.) depending on if the tip has graduations on it and if you need to blow out the last drop or not. These pipets are graduated so that you can deliver a variable volume of solution.

Figure 1 Various types of pipets (pipettes) (a) 10 mL Mohr pipet, (b) 10 mL Serological pipet,

and (c) 25 mL volumetric pipet

https://welfarejambo.blogspot.com/2017/06/measuring-and-dispensing-liquid_62.html

If using a Mohr pipet, you must record the initial and final volume to obtain the volume transferred and the volume cannot go below the last graduation mark on the pipet (see Figure 1(a)). Notice that the markings start at 0 and INCREASE as you move down the pipet toward the tip. There are also no graduation lines at the tip of this pipet. See Figure 2 to see how to deliver 3.20 mL of solution. Note that you must record the initial and final volumes in your notebook to obtain the volume delivered.

Figure 2 A 10 mL Mohr pipet and how to deliver 3.20 mL of solution

http://www2.hawaii.edu/~johnb/micro/m140/graphics/serological_pipette_tip.jpg

Your lab drawer contains a 25-mL Mohr pipet.

It is always best to have the volume measurements fall within the limits of the graduation mark(s). In other words, it is best to start the initial volume just below the first graduation mark as you can estimate best if there are graduation marks above and below the meniscus and graduation marks above and below the meniscus for the final volume.

Figure 1(b) shows a picture of a serological pipet. This pipet can be identified by the double wide marks at the top of the pipet and the graduation lines on the tip. When using a serological pipet, you must blow out or force the last drop out of the pipet if you let the solution completely drain from the pipet. If you are delivering a volume between two measurement lines, you do not have to worry about blowing out the last drop. The labs in chemistry do not use any serological pipets.

Another type of graduated pipet is found in your lab drawer and it has graduation lines on the tip and NO double wide lines at the top (see Figure 3). Note that the volume starts at 10 and decreases down as you move toward the tip. This is the ISO 835 Blau brand graduated pipet. This type of graduated pipet is easier to use as you only need to set the meniscus one time and you do not have to blow out the last drop.

Figure 3 A 10-mL graduated pipet and how to deliver 3.00 mL of solution.

https://www.brand.de/fileadmin/_processed_/csm_typ213_E_0cc3ca536f.jpg

In this experiment, we will be using a 10-mL ISO 835 Blau Brand graduated pipet.

As you can see, you have to first look at your pipet carefully before you use it so that you know HOW to use it correctly. Take a moment to carefully look at your pipets how they are marked.

ACCURACY AND PRECISION

Accuracy and precision are necessary to determine how confident one can be in the results obtained by experiment. Students often confuse the terms accuracy and precision. Accuracy determines how close you are to the true value. If the density of lead is known to be 11.6 g/mL and you obtain a density of lead by experiment and get 6.8 g/mL, what does it tell you about the accuracy of the measurement? The experimental density is a bit off from the known density. One way to show the level of accuracy is to calculate a percent error. Percent error is defined as

Percent error = | experimental value – true or known value | X 100%Equation 1

True or known value

where the “|” symbols represent absolute value or a positive answer. Some instructors prefer not to use absolute symbol signs so that the value can be reported as positive or negative. One benefit of doing this is that it helps to show systematic error. Systematic error is error that can be identified and minimized and which is consistently in the same direction. For example, if your percent error is consistently negative for a particular result, then it shows that your results are consistently below the true value. This might indicate that there is something wrong with your approach or measurement tool. For example, if your balance is not calibrated correctly (consistently measuring a smaller mass) then your result might yield a consistently negative error if your result uses mass. You might ask your instructor how he/she would like you to report percent error as each instructor has their own preference. Random error cannot be eliminated as it is inherent in the tools and/or techniques used in the experiment. Random error can be improved, however, by using more precise measurement tools. If you are doing multiple trials in an experiment, it is common to use your average value of your final results in the percent error calculation rather than the result of one trial if using absolute value for percent error. When determining if systematic error exists, it is common to use each trial’s result to calculate percent error to identify any of these types of errors. Again, ask your instructor what he/she prefers you to use for the percent error calculation.

Precision is defined as the reproducibility of the measurement, i.e. how close the results of multiple trials are to each other. Sometimes you might hear someone refer to the precision of a measuring tool. This refers to how well you can measure a quantity with a particular measurement tool. If one were to estimate a volume measurement as 6.69 mL it implies that it can be measured to 6.69 + 0.01 mL. The + 0.01 portion of the measurement shows the level of precision of the tool, or to what decimal place should be reported in the measurement. Therefore, a measurement of 6.68 mL or 6.70 mL would be acceptable as it is within the precision of the tool since the last decimal place is estimated to plus or minus 0.01. Since the last decimal place is uncertain, be careful not to overestimate or underestimate the value. This can be minimized by looking at the bottom of the meniscus at eye level for volume measurements.

If you thought that the bottom of the meniscus was right on the line, you would report the measurement as 6.70 mL and NOT 6.7 mL. By looking at 6.7 mL it implies that the tool that was used to measure the volume could only be estimated to + 0.1 which we know is not true. Therefore, the zero is added to the number to show that this tool can estimate volume to + 0.01 mL. Note that 6.70 mL and 6.69 mL both have 3 significant figures since the volume was measured with the same tool. Therefore, when reporting measurements where the volume might fall exactly on a line, you must still report the proper number of decimal places that represents the estimated digit in the measurement by adding zeros.

To quantify the level of precision or reproducibility of multiple measurements, you might use the range, the deviation (or percent deviation) or standard deviation (or percent relative standard deviation) about the mean or average. The meanings of these can be found in detail on a separate handout on precision and accuracy provided by your instructor or he/she may go these with you as part of your prelab lecture. Your instructor will tell you how he/she would like you to report the accuracy and precision of your results obtained from your experiment.

PRELAB QUESTIONS:

1. Determine the initial volume in this 10 mL graduated Mohr pipet.

http://faculty.sdmiramar.edu/fgarces/LabMatters/ChemTech/modules/pipetmohr/PIC/00464100.JPG

2. When a piece of metal weighing 14.80 g was placed into a graduated cylinder initially containing 14.0 mL of water, the water level rose to 20.2 mL. What is the density of the metal using units of g/mL? Use Table 1 to determine the identity of the metal and calculate the percent error. Be sure to use the correct number of significant figures in your final answers.

Table 1: Densities of Different Metals at Room Temperature

Metal

Density (g/cm3)

Aluminum

2.70

Brass

8.40 - 8.73*

Copper

8.94

Gold

19.3

Iron

7.86

Lead

11.34

Magnesium

1.74

Silver

10.5

Tin

5.75

Zinc

7.14

*Depends on the relative amounts of copper and zinc.

CHEMICALS AND MATERIALS

Part 2.

Part 3. Part 4

Distilled water

10-mL volumetric pipet

Seawater (saline solution) unknown metal

10-mL graduated pipet 25 mL graduated

cylinder

EQUIPMENT TO BE CHECKED OUT FROM CART

Pipet bulb (Parts 1, 2, and 3)

Unknown metal

25 mL graduated cylinder

SAFETY

Goggles must be worn in the laboratory.

Closed toed shoes

WASTE DISPOSAL

Dispose of all solutions into the sink

EXPERIMENTAL PROCEDURE:

Part 1: Practice with pipets (individuals, no pairs)

1. Check out a pipet bulb from your instructor and make sure it is clean and dry before you use it. You can check it by tapping it onto a dry paper towel. If fluid comes out, separate the bulb from the Teflon piece and rinse both with distilled water. Tap both pieces on a dry paper towel until no more liquid comes out.

2. Obtain a beaker and place approximately 50 mL of distilled (DI) water into it. Label it as “Beaker 1” using tape from the tape dispenser from the instructor’s bench.

3. Practice delivering 10 mL of water using a pipet bulb using the 10 mL volumetric pipet. The following are the steps needed in preparing and using pipets:

a. You will need to first condition the pipet with the solution you are going to use. This is needed so that you do not contaminate or dilute your solution when pipetting your desired solution.

i. Squeeze the pipet bulb and place it gently onto the end of the pipet.

ii. Place the tip of the pipet gently into the bottom of the beaker containing the 50 mL of DI water (the solution that you will be using for this experiment).

iii. Slowly release the pressure on the bulb and pull up a small amount of the DI water.

iv. Move the pipet and place the tip into an empty beaker (Label it as “Beaker 2”) for your rinse solutions. Place the tip on the bottom of the beaker and quickly remove the bulb and put your forefinger (not your thumb!) over the end of the pipet.

v. Holding the pipet almost horizontal, rotate the pipet back and forth and up and down while quickly releasing your finger periodically so that the DI water wets the complete interior of the pipet and slightly past the graduated mark while keeping the tip over the beaker so that the rinses drain from the tip into the beaker. Do not allow any of the DI water to come out the top of the pipet.

vi. Let the rest of the solution drain from your pipet and touch off the tip of the pipet against the inside of Beaker 2 to get the last drop removed.

vii. If there are droplets hanging onto the inside of the pipet, you may need to clean the pipet with a detergent solution. Clean by pulling up soapy water a few times from a separate beaker, then rinse with tap, then rinse with DI water. If no more droplets are hanging on the inside of the pipet, then the pipet is clean.

viii. If the pipet is clean, repeat the conditioning step two more times using the DI water from Beaker 1.

ix. It is very important not to contaminate your original solution (Beaker 1) with any rinses, etc.

x. Make sure the pipet bulb is free from DI water.

b. After conditioning the pipet, you will need to deliver 10 mL of DI water.

i. Squeeze the pipet bulb and place it gently on the end of the pipet.

ii. Place the tip of the pipet gently at the bottom of Beaker 1 containing the DI water.

iii. Slowly release pressure from the pipet bulb to slowly pull up the DI water. Pull enough DI water so that the level is above the calibration line.

iv. With the pipet tip gently pressed at the bottom of the beaker (to slow the flowrate), remove the bulb and replace with your forefinger.

v. Hold up the pipet with one hand and the beaker with the other hand until the graduation mark is at eye level.

vi. Bring up the tip of the pipet and angle the beaker so that the tip of the pipet is touching the SIDE of the beaker and out of the solution.

vii. Slowly release pressure from your finger until the bottom of the meniscus touches the graduation mark at eye level. If you are below the graduation mark, you will need to redo it.

viii. Move the pipet to your receiving vessel so that the tip is against the inside wall of the receiving vessel.

ix. Let the DI water drain by gravity. Once drained, wait 5 seconds then touch off the last drop by wiping the tip against the inside wall of the receiving vessel.

x. DO NOT blow out the last drop! Volumetric pipets are designed to leave a little bit of solution/DI water in the tip.

xi. The volume delivered is 10.00 mL.

c. Make sure the pipet bulb is still free from DI water.

4. Practice delivering approximately 8 mL of distilled water using the 10 mL graduated pipet. Note the markings on the pipet. They go from 10 at the top and are marked with graduation marks all of the way to the tip. For the 10 mL graduated pipet, you will need to record the initial volume.

a. Follow the guidelines in step 3 above for conditioning and pulling up solution into the pipet.

b. Pull up DI water to just past the 8 mL mark. Remove the pipet bulb, replace with yoru forefinger and let the DI water drain until it is at the 8 mL mark.

c. Move the pipet to your receiving vessel and record your initial volume to the correct number of significant figures.

d. Remove your finger and let the DI water drain completely by gravity, wait 5 seconds, and then touch off the tip on the inside of the receiving vessel.

e. The volume delivered is the initial volume recorded.

5. Ask your instructor for help if needed. Your results will only be as good as your pipetting technique when using pipets! After you feel comfortable using the pipet or at the direction of your instructor, continue to part 2 of the experiment.

Part 2: Determination of the density of distilled water (Individuals, no pairs)

1. Place about 50 mL of distilled water into a labeled 100 or 150 mL clean beaker.

2. Condition a 10 mL volumetric pipet with distilled water. Pour rinses into a separate container.

3. Obtain the smallest clean, dry beaker from your drawer (50 or 100 mL).

4. Tare the balance by pushing the “tare” button. The readout should show 0.000g (always tare the balance before using it). Weigh the dry beaker, bring the beaker back to your bench area to pipet 10 mL of distilled water (not from the rinses used to condition it!) into the beaker, and reweigh the mass of both water and beaker after you tare the balance again. Be sure to record your initial and final mass measurements. Be sure to use the same balance for each part of the experiment as this will yield more consistent results.

5. Be sure to record the correct volume and mass for this trial.

6. Repeat part 2 so that you have a total of 5 trials. If you think you might have messed up a trial and if there is time, repeat for an additional trial. Be sure to record all data for all trials, however.

7. Place a thermometer in the beaker with the distilled water and record the temperature of the distilled water to the proper number of significant figures.

8. Determine the density of distilled water.

9. Obtain the true density of water at this temperature and record using Table 2 below (Note that every one degree difference in temperature is listed down the first column of the table and every tenth of a degree is listed in the first row of the table. Therefore the density of water at 15.9oC is 0.998959 g/mL and is found by first searching the nearest whole degree, 15, down the first column and then searching to the nearest tenth degree, 0.9, as you go across the first row of the last column and read down the “0.9” column).

Table 2: Density of Water (g/mL) vs. Temperature (°C)

Temp.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

15

0.999099

0.999084

0.999069

0.999054

0.999038

0.999023

0.999007

0.998991

0.998975

0.998959

16

0.998943

0.998926

0.998910

0.998893

0.998877

0.998860

0.998843

0.998826

0.998809

0.998792

17

0.998774

0.998757

0.998739

0.998722

0.998704

0.998686

0.998668

0.998650

0.998632

0.998613

18

0.998595

0.998576

0.998558

0.998539

0.998520

0.998501

0.998482

0.998463

0.998444

0.998424

19

0.998405

0.998385

0.998365

0.998345

0.998325

0.998305

0.998285

0.998265

0.998244

0.998224

20

0.998203

0.998183

0.998162

0.998141

0.998120

0.998099

0.998078

0.998056

0.998035

0.998013

21

0.997992

0.997970

0.997948

0.997926

0.997904

0.997882

0.997860

0.997837

0.997815

0.997792

22

0.997770

0.997747

0.997724

0.997701

0.997678

0.997655

0.997632

0.997608

0.997585

0.997561

23

0.997538

0.997514

0.997490

0.997466

0.997442

0.997418

0.997394

0.997369

0.997345

0.997320

24

0.997296

0.997271

0.997246

0.997221

0.997196

0.997171

0.997146

0.997120

0.997095

0.997069

25

0.997044

0.997018

0.996992

0.996967

0.996941

0.996914

0.996888

0.996862

0.996836

0.996809

26

0.996783

0.996756

0.996729

0.996703

0.996676

0.996649

0.996621

0.996594

0.996567

0.996540

27

0.996512

0.996485

0.996457

0.996429

0.996401

0.996373

0.996345

0.996317

0.996289

0.996261

28

0.996232

0.996204

0.996175

0.996147

0.996118

0.996089

0.996060

0.996031

0.996002

0.995973

29

0.995944

0.995914

0.995885

0.995855

0.995826

0.995796

0.995766

0.995736

0.995706

0.995676

30

0.995646

0.995616

0.995586

0.995555

0.995525

0.995494

0.995464

0.995433

0.995402

0.995371

(from Handbook of Chemistry and Physics, 53rd Edition, p. F4)

Part 3: Determine the density of a seawater sample (saline solution) (Individuals, no pairs):

1. Place about 50 mL of seawater into a labeled 100 or 150 mL clean, dry beaker.

2. Condition a 10 mL graduated pipet (ISO 835) with seawater and put the rinses into a separate labeled container.

3. Obtain the smallest clean, dry beaker in your drawer (50 or 100 mL).

4. Weigh the dry beaker and then pipet about 8 mL of seawater into the beaker and reweigh. Be sure to record the actual volume to the proper number of significant figures and be sure to use the same balance for each part of the experiment as this will yield more consistent results.

5. Be sure to record the correct volume and mass for this trial.

6. Repeat part three so that you have a total of 5 trials. If you think you might have messed up a trial and if there is time, repeat for an additional trial. Be sure to record all data for all trials, however. Pour the seawater into the sink when finished.

7. Place a thermometer in the beaker with the seawater and record the temperature of the seawater to the proper number of significant figures.

8. Determine the density of seawater.

9. Obtain the true density of seawater at this temperature and record using Table 3 below:

Table 3: Densities of Saline Solutions at Various Temperatures

(Source: C. E. Boyd, Water Quality in Ponds for Aquaculture; 1990)

Part 4: Determination of the density of an Unknown Metal (Individuals, no pairs)

1. Obtain an unknown metal cylinder from your instructor. Record the number of the unknown that is stamped on one end of the cylinder and record it directly in your lab notebook otherwise your instructor will not be able to grade this portion if your unknown number is not recorded.

2. To keep the balances clean, it is always a good idea to use your weigh boat in your drawer. Tare the balance first by pushing the “tare” button. The balance should read 0.000 g. Obtain the mass of your metal cylinder either by mass difference (weigh the boat, record the mass, put the metal in the weigh boat and reweigh and determine the mass of the metal by the difference in the two measurements) or weigh by taring (tare the weigh boat so the mass reads 0.000g, place your metal in the weigh boat to obtain the mass of the metal directly). Make sure you place the cylinder on one end so that it does not roll around in the weigh boat!

3. Fill your 25 mL graduated cylinder half full of water (tap is fine). Record the initial volume of water to the correct number of significant figures.

4. Tilt the graduated cylinder at an angle and slowly slide the metal down the side of the graduated cylinder. If you break the graduated cylinder, you will receive a grade deduction on your lab report so be careful!!

5. After placing the metal in the graduated cylinder, tap the metal so that there are no air bubbles clinging to the metal. Measure the final volume of water in the graduated cylinder to the proper number of significant figures. The volume of the metal is then obtained by volume displacement.

6. From your mass and volume of the metal, determine the density of the metal.

7. Repeat the procedure for this part for a total of 5 trials. If you think you might have messed up a trial, repeat for another trial but do not erase or delete any of the data from any of the trials.

DATA ANALYSIS:

Part 2: Determination of the density of distilled water (Individuals, no pairs)

1. Using the mass of water and the volume of water, calculate the density of water in units of g/mL and in units of g/cm3.

2. Calculate the average value for the density of water.

3. Using Table 2 for density of water, find the true density at the temperature you measured in the lab.

4. Calculate the percent error.

5. Using either % deviation or % relative standard deviation, determine the precision of the experiment (ask your instructor which he/she prefers).

Part 3: Determine the density of a seawater sample (saline solution)

1. Using the mass of water and the volume of seawater, calculate the density of seawater in units of g/mL and in units of g/cm3.

2. Calculate the average value for the density of seawater.

3. Using Table 3 for density of seawater, find the true density at the temperature you measured in the lab.

4. Calculate the percent error.

5. Using either % deviation or % relative standard deviation, determine the precision of the experiment (ask your instructor which he/she prefers).

Part 4: Determination of the density of an Unknown Metal

1. Using the mass and volume of the metal, determine the density of water in units of g/mL and g/cm3.

2. Calculate the average value for the density of the metal cylinder.

3. Using Table 1 for densities of different metals, find the identity of your metal using the average value of your density and the value in the table that is closest to this value.

4. Calculate the percent error.

5. Using either % deviation or % relative standard deviation, determine the precision of the experiment (ask your instructor which he/she prefers).

POSTLAB QUESTIONS:

1. By comparing the densities of water and seawater, explain why a person might float better in the ocean as compared to a freshwater lake.

2. Comment on the magnitude of the precision and accuracy for each part of the experiment.

3. Identify sources of error that may have occurred during this experiment. Please do not use “human error” as a source of error.

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