CHEM 181 #1 Basic Laboratory Operations Fall 2020 PDF

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CHEM 181 Experiment #1

CHEM 181: Experiment #1 Basic Laboratory Operations: Accuracy & Precision and Density Objectives    

To develop the skill for properly operating a balance To determine which glassware is more accurate and/or precise To develop the technique of using glassware To determine the density of an unknown substance

Introduction Much of what is done in the chemistry laboratory involves taking measurements. A measurement is a quantitative observation that has both a numerical value and a unit. How well a measurement is taken determines both its precision and its accuracy. Precision is related to the reproducibility of the measurement. It is a comparison of several measured values obtained in the same way. For example, a student measures the volume of a liquid three times, obtaining the values 3.66 mL, 3.62 mL, and 3.64 mL. These measurements have high precision; there is only a 0.040-mL difference between the highest and lowest values. Accuracy is a comparison of a measured value to the theoretical, accepted, or true, value. Suppose the volume of the liquid had actually been 5.44 mL. Then the accuracy of the three measurements taken would be low because a difference of 1.80 mL between the average measured value and the true value is relatively large. In order to properly assess the value of a measurement, a percent error is often calculated.

percent error=

experimental−theoretical x 100 theoretical

This experiment will also explore density. Density is an intensive property that expresses the mass of a substance per unit volume. In the English system, the density of water at 4°C is 8.34 lb/gal or 62.2 lb/ft3, whereas in SI the density of water at 4°C is 1.00 g/cm3 or 1.00 g/mL.

density=

mass of sample volume of sample

In this experiment, the density of an unknown water-insoluble solid will be determined using the displacement of water.

MATERIALS Metal Cylinders 10 mL Volumetric Pipet (this and flask is much better) Pipet pumps 50 mL Volumetric Flask 15% NaCl solution (homogenous) 50 mL Beaker 50 mL Graduated Cylinder (more precise and accurate than beaker)

Experimental Procedure A. Accuracy and Precision Obtain about 60 mL of 15.0% NaCl solution in your 100-mL beaker. Be sure to record your data in the table on your report sheet. Use the same balance for all measurements. Also, record all the digits reported on the balance and the proper significant figures for each type of glassware! 1. Weigh a 50-mL beaker that has the graduate markings using the balance. 2. Add the NaCl solution to the beaker using the graduated markings to measure a volume of 20. mL. (2SF) 3. Weigh the beaker and solution. 4. Repeat steps 1-3 for a total of 3 trials. 5. Weigh a 50-mL graduated cylinder using the balance. 6. Add the NaCl solution to the cylinder to measure a volume of 20.0 mL. (3 SF) 7. Weigh the cylinder and solution. 8. Repeat steps 5-7 for a total of 3 trials. 9. Weigh a 50-mL volumetric flask. 10. Add the NaCl solution to the volumetric flask to the 50.00 mL mark.

11. Weigh the flask and solution.

12. Repeat steps 9-11 for a total of 3 trials.

13. Weigh a 50-mL beaker using the balance.

14. Using a 10-mL volumetric pipet, add 10.00 mL of the NaCl solution to the 50-mL beaker.

15. Weigh the beaker and solution. 16. Repeat steps 13-15 for a total of 3 trials.

B. Density 1. Obtain an unknown metal solid and record its number. Using the balance, tare the mass of a piece of weighing paper, place the solid on the weighing paper and measure its mass. Record the mass. Do not touch the metal again with your fingers. Use the weigh paper to hold the metal. 2. Half-fill a 50-mL graduated cylinder with water and record its volume.

3. Gently slide the known mass of solid into the graduated cylinder held at a 45° angle. Roll the solid around in the cylinder, removing any air bubbles that are trapped or that adhere to the solid. Record the new water level. The volume of the solid is the difference between the two water levels. 4. Repeat the procedure with two additions samples of the same metal. 5. Record your data.

Adapted from: Conceptual Chemistry Laboratory Manual. D. Gibson and J. Suchocki. 3rd edition. Pearson, 2007. Laboratory Manual for Principles of General Chemistry. J. Beran. 9th edition. Wiley, 2011.

Basic Laboratory Operations Prelaboratory Assignment Answers the following questions in your lab notebook. Title it Pre-Lab and the experiment number. 1. Diagram (or describe) the cross section of a graduated cylinder, illustrating how to read the meniscus. The graduated cylinder needs to be at eye level. Then you need to read the point in the water where it dips, the lowest point of the curve.

2. Why is it necessary to obtain more than one trial when measuring out the sodium chloride solution for each piece of glassware? It is necessary to obtain more than one trial when measuring out the sodium chloride solution for each piece of glassware because the more measurements you make, the more precision measurements you will have, and the smaller the percent error will be.

3. Please look up information on volumetric pipets for the following questions:

a. Remove the drop suspended from a pipet tip by… Touching it to the wall of the waste beaker

b. The finger used to control the delivery of liquid from a pipet is the… Index Finger

c. What should be done with the last bit of liquid remaining in the pipet after delivery?

Dispose the unknown liquid and the rinses in the waste liquid container (acid waste or salt waste)

4. The density of gold is 19.31 g/mL, and the density of platinum is 21.43 g/mL. If equal masses of gold and platinum are transferred to equal volumes of water in separate graduated cylinders, which graduated cylinder would have the greatest volume change? Explain or show with a sample calculation. Density= Mass/Volume Mass= Volume x Density (assume mass = 10g) Platinum: 10g=V x 21.43g/mL , 10g/21.23g/mL=V , 0.4666355576mL = V Gold: 10g=V x 19.31g/mL , 10g/19.31g/mL=V , 0.5178663905mL = V The cylinder with the greatest volume change will be the cylinder containing the gold because the gold has a larger volume than the platinum. I also know this because the object with the lower density will displace the most water, and in this instance that object is gold.

5. The mass of a beaker is 5.333 g. After 5.00 mL of a concentrated hydrochloric acid solution is pipetted into the beaker, the combined mass of the beaker and the hydrochloric acid sample is 11.229 g. From the data, what is the measured density of the hydrochloric acid solution? (3SF) Mass of acid: 11.229-5.333=5.896 g Density= Mass/Volume Density= 5.896g/5mL Density=1.1792= 1.18g/mL

Basic Laboratory Operations Report Sheet Perform the following experiment and collect all of your data in your lab notebook following the example charts below. Remember to be neat.

Show All Calculations Part A: Accuracy and Precision

Theoretical Density of 15% NaCl = 1.112 g/mL

Beaker

Mass of empty 50mL beaker (g) Mass of beaker + liquid (g) Mass of liquid (g) Volume of liquid (mL) Density of liquid (g/mL)

Trial 1

Trial 2

Trial 3

32.822g

32.823g

32.822g

54.371g

54.285g

53.733g

54.371g-32.822g= 21.549g

54.285g-32.823g= 21.462g

20. 21.549g/20.mL= 1.077g/mL

20. 21.462g/20.mL= 1.073g/mL

Average density of liquid (g/mL)

53.733g-32.822g= 20.911g

20. 20.911g/20.mL= 1.046g/mL

1.1g/mL

(1.077+1.073+1.046)/3 = 1.065

Standard deviation (g/mL)

0.017g/mL

1.077-1.065=0.012, 1.073-1.065=0.008, 1.046-1.065=0.019 √((.0122+.0082+.0192)/2 = 0.0168

Percent error (%)

4.2%

Calculate this using the average Density and the Theoretical Density. |A-E|/A x 100 |1.112-1.065|/1.112 x 100 =

Graduated Cylinder (3sf) Trial 1 19.612g Mass of empty 50mL grad. cylinder (g) 40.960g Mass of grad. Cylinder + liquid Mass of liquid (g) 40.960g-19.612g= 21.348g

Volume of liquid (mL) Density of liquid (g/mL)

20.0 21.348g/20.0mL= 1.0674g/mL

Trial 2

Trial 3

19.801g

19.737g

40.458g

40.122g

40.458g-19.801g= 20.657g

20.0 20.657g/20.0mL= 1.03285g/mL

40.122g-19.737g= 20.385g

20.0 20.385g/20.0mL= 1.01925g/mL

Average density of liquid (g/mL) (1.0674+1.03285+1.01925)/3 = 1.03983

1.04g/mL

Standard deviation (g/mL)

.0248g/mL

1.0674-1.03983=0.02757, 1.03285-1.03983=0.00698, 1.01925-1.03983=0.02058 √((0.027572+0.006982+0.020582)/2)=0.0248229904

Percent error (%)

6.49%

Calculate this using the average Density and the Theoretical Density. |A-E|/A x 100 |1.112-1.03983|/1.112 x 100= 6.490107914

Volumetric Flask (4sf) Trial 1 36.089g Mass of empty 50mL flask (g) 90.370g Mass of flask + liquid (g) Mass of liquid (g) 90.370g-36.089g= 54.281g Volume of liquid 50.00 (mL) 54.281g/50.00mL= Density of liquid 1.08562g/mL (g/mL)

Trial 2

Trial 3

36.253g

36.242g

90.351g

90.416g

90.351g-36.253g= 54.098g

50.00 54.098g/50.00mL= 1.08196g/mL

Average density of liquid (g/mL)

90.416g-36.242g= 54.174

50.00 54.174g/50.00mL= 1.08348g/mL

1.084g/mL

(1.08562+1.08196+1.08348)/3=1.083687g/mL

Standard deviation (g/mL)

.001859g/mL

1.08562-1.083687=0.001933, 1.08196-1.083687=0.00177, 1.08348-1.083687=0.000207 √((0.0019332+0.001772+0.0002072)/2)=0.001859064

Percent error (%)

2.546%

Calculate this using the average Density and the Theoretical Density. |A-E|/A x 100 |1.112-1.083687|/1.112 x 100=2.546133094

Volumetric Pipet (4sf)

Mass of empty 50mL beaker (g) Mass of beaker + liquid (g) Mass of liquid (g) Volume of liquid (mL) Density of liquid (g/mL)

Trial 1

Trial 2

Trial 3

32.827g

32.863g

32.841g

43.719g

43.803g

43.730g

43.719g-32.827g= 10.892g

10.00 10.892g/10.00mL= 1.0892g/mL

Average density of liquid (g/mL)

43.803g-32.863g= 10.94g

10.00 10.94g/10.00mL= 1.094g/mL

43.730g-32.841g= 10.889g

10.00 10.889g/10.00mL= 1.0889g/mL

1.091g/mL

(1.0892+1.094+1.0889)/3=1.0907

Standard deviation (g/mL) 1.0892-1.0907=0.0015, 1.094-1.0907=0.0033, 1.0889-1.0907=0.0018 √((0.00152+0.00332+0.00182)/2)=0.0028618176

.002862g/mL

Percent error (%)

1.915%

Calculate this using the average Density and the Theoretical Density. |A-E|/A x 100 |1.112-1.0907|/1.112 x 100= 1.915467626

Part B: Density: Sample number: ______________ Trial 1

Trial 2

Trial 3

Mass if Sample (g)

49.238g

49.020g

49.125g

Initial water volume (mL) Final water volume (mL) Volume of sample (mL) Density of sample (g/mL)

25.0mL

25.0mL

25.0mL

31.0mL

31.0mL

31.5mL

31.0mL-25.0mL= 6mL

31.0mL-25.0mL= 6mL

31.5mL-25.0mL= 6.5mL

49.238g/6mL= 8.206333g/mL

49.020g/6mL= 8.17g/mL

49.125g/6.5mL= 7.55769g/mL

Average density of sample (g/mL)

7.98g/mL

(8.206333+8.17+7.55769)/3=7.978007667

Standard deviation (g/mL)

0.364g/mL

8.206333-7.978007667=0.2283253, 8.17-7.978007667=0.1919923, 7.55769-7.978007667=0.420317667 √((0.22832532+0.19199232+0.4203176672)/2)=0.3644587954

CAUTION: Use your own words. Do not copy from your neighbor or the internet. That is plagiarism and you will have to be reported. 1. From your data, which type of glassware is most precise (or has the least amount of uncertainty)? (lower SD = more precise) From my data, the type of glassware that is most precise is the volumetric flask because it has the lowest standard deviation of .001859g/mL.

Which do you think should have had the least uncertainty? I think the volumetric pipet should have had the least uncertainty.

Why? I think this because the volumetric pipet has very precise measurements of the cylinder, which means the measurement would be very accurate and precise. I also feel like the pipet has an easier way of controlling the liquid into the cylinder

2. From your data, which type of glassware is most accurate (or has the lowest percent error)? (the lower the % error, the more accurate)

From my data, the glassware that is most accurate is the volumetric pipet because it has the lowest percent error of 1.915%.

Which do you think should have been the most accurate? I agree with which glassware was the most accurate in the experiment, which was the volumetric pipet.

Why? The volumetric pipet is the most accurate in my opinion because the measurements are very precise. Since the measurements are very precise, there is a higher chance of calculating the most accurate measurements of a substance.

3. Explain why a highly precise measurement does not always mean a highly accurate one. Precision refers to answers that are very specific, which means it has the ability to measure things down to very small increments. This typically reflects a low error because of how precise the measurements are. Accurate refers to and answer that is close to a true answer. Because experiments have more than one trial, all the measurements taken can be precise to each other, meaning they are all relatively the same, but they may not be accurate to the designated true number.

4. Suggest ways you could have improved your precision. To improve your precision in the lab, you can pay closer attention to detail by using equipment properly and increasing sample size. Before completing the lab, you should also make sure your equipment is properly calibrated, function, and clean.

5. Explain, in detail, why the density of a substance is independent of sample size? The density of a substance is independent of sample size. The definition of density is: Density=Mass/Volume. This means that if the sample size increases, meaning the volume increases, then so does the mass. But the density will remain the same. In other words, density is a function of mass and volume, if it remains constant, then either both or neither (mass and volume) can change.

6. In Part A, suppose that after delivery several drops of the water cling to the inner wall of the pipet (because the pipet wall is dirty). How, in terms of calculations, will this technique error affect the calculated density? Will it increase or decrease and why? If water droplets cling to the inner wall of the pipet, it will cause the volume of the water to be a lower value than what the pipet is calibrated to, therefore making the density of the water higher. This is because I would have the same measured amount of mass, but the volume will be lower since some of it remains on the walls of the pipet. When the mass is the same but the volume is lower, I will have a greater density reported.

7. In Part B, the metal sample is not completely submerged in the water. How, in terms of calculations, will this technique error affect the calculated density? Will it increase or decrease and why? This technique error will not affect the mass of the solid, but will affect the volume by decreasing it. Since the formula is Density=Mass/Volume, by decreasing the volume you will result in

calculating an increase in density. This is because you are dividing the mass by a smaller number....