Post lab 2.docx - Precision and Accuracy in Scientific Glassware Post-Lab Draft with Extra Data PDF

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Precision and Accuracy in Scientific Glassware Post-Lab Draft with Extra Data...

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Evaluating the precision and accuracy of scientific glassware – Postlab Report

First & last name

Lab section

1411-123

Lab partner(s)

Date

9/24/2017

Print and complete the following postlab assignment using the information recorded in your lab notebook. Show all work and calculations for each problem to receive credit. All numerical answers must be clearly indicated with a circle or a box around the final answer, contain the proper number of significant figures, and include appropriate units. Written answers must be in complete sentences, use proper grammar, and have correct spelling. Unclear answers may result in loss of credit. Accuracy and precision of scientific glassware 1. What was the temperature of the DI water in the laboratory? 22.6 oC 2. What is the density of water at this temperature (see table 1 in the experiment document)? 0.9975415 g/mL 3. Create a computer-generated table that contains the measurements for each piece of scientific glassware. This can be done using either MS Word or Excel (or equivalents). The title row of the table and one piece of glassware is provided as an example (do not include these sample values in your table). Add rows for the remaining pieces of glassware and the pertinent measurements and calculated values. Be sure all values have the correct precision (number of decimal places), and that the table is labeled and has an appropriate title. Notice the units are at the top of each column – this means the unit does not need to be (and should not be) written in individual cells. Print and attach the table. This is an exercise in creating computerized data tables, and hand-drawn tables will not receive credit. Volume of Mass of Mass of Volume of water container + empty water Mass of Glassware (actual) water container (theoretical) water size and (g) (g) (mL) (g) type Trial (mL) 1 37.3571 47.3668 10.0097 10.00 10.03 10 mL graduated cylinder

2 3 Average

The actual volume of water is the measured volume of water (using the graduations on the glassware). The theoretical volume of water is the volume calculated using the measured mass and density. 4. Show a sample calculation for the theoretical volume of water. Mass of water: 90.9822g;

Density: 0.9975415 (g/mL) 1

Volume =

m D

=

90.9822 0.9975415

= 91.2064 (mL)

5. Complete the following table (fill this table in, including a title, you do not need to recreate it on the computer). Use the average volumes of water from previous table for each glassware type. Table. Percent error and Standard deviation of experiments Volume of water Glassware size and Volume of water type (theoretical) (actual) (mL) (mL) 125 mL Erlenmeyer 100.0 91.5611 Flask (measure 100mL) 150 mL Beaker 100.0 96.7144 (measure 100 mL) 100.0 98.74 100 mL graduated cylinder (measure 100mL) 10.0 9.79 10 mL graduated cylinder (measure 10mL) 50 mL Burret (measure 10.0 9.9393 10 mL) 10 mL Volumetric 10.0 10.0523 pipet (measure 10mL)

Percent error (%)

Standard deviation

8.4389

2.9049

3.2856

1.8126

1.26

1.08

2.1

0.46

0.6070

0.2464

0.5230

0.2287

6. Show a sample calculation for the percent error for one piece of glassware. See appendix B for information about percent error.

Percentage Error =

| your result−accepted value| accepted value

×100

150 mL Beaker:

Percentage Error =

|96.7144 −100.0| 100.0

×100

= 3.2856 %

7. Show a sample calculation for the standard deviation for one piece of glassware. 2

Standard deviation =

√

√

∑ |Theorical volume− Actual volume| Number of experiments

|91.2064−100.0|+|91.7972−100.0|+|91.6797 −100.0| 3

= 2.9050

8. Rank the pieces of glassware used to measure 100 mL of water in order of most accurate to least accurate. Explain how you decided this order. 100 mL graduated cylinder, 150 mL Beaker ,125 mL Erlenmeyer Flask. The order depends on the percent errors.

9. Rank the pieces of glassware used to measure 100 mL of water in order of most precise to least precise. Explain how you decided this order. 100 mL graduated cylinder, 150 mL Beaker ,125 mL Erlenmeyer Flask. The order depends on the standard deviation. 10. Rank the pieces of glassware used to measure 10 mL of water in order of most accurate to least accurate. Explain how you decided this order. 10 mL Volumetric pipet, 50 mL Burret, , 10 mL graduated cylinder The order depends on the percent errors. 11. Rank the pieces of glassware used to measure 10 mL of water in order of most precise to least precise. Explain how you decided this order. 10 mL Volumetric pipet, 50 mL Burret, 10 mL graduated cylinder. The order depends on the standard deviation.

12. For the following questions, decide which piece of glassware is most appropriate, and suggest an approximate size. Indicate why this is the best piece of glassware and size to use and be sure to cite accuracy and precision as necessary.

a. A student wants to create a boiling water bath using around 250 mL of water.

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We should use 125 mL Erlenmeyer Flask because we can boil the 125 mL Erlenmeyer Flask with 125 mL of water twice to get around 250 mL of boing water. We don’t need exactly volume so it’s most appropriate.

b. A student wants to measure out exactly 25.00 mL of water to make a solution with a specific concentration. Using 50 mL Burret because the 50 mL Burret has high accurate and high precision (not the best in the experiments) but the maximum volume of Burret is greater than Volumetric pipe (50 > 25 > 10).

c. A student requires between 23 – 24 mL of a liquid for a chemical reaction. Using 50 mL Burret because the 50 mL Burret has high accurate and high precision (not the best in the experiments) but the maximum volume of Burret is greater than Volumetric pipe (50 > 23, 24 > 10). 13. A student is writing methods for the following experimental procedures: “combine 10.0 mL of an NaOH solution, 5.00 g of methyl salicylate, and 25.0 mL of ethanol in a flask. Heat the flask in a boiling water bath for 10 minutes with continuous stirring.” The student decides to use a 10-mL graduated cylinder to measure the NaOH solution, a 20mL Erlenmeyer flask to hold the reaction mixture, and a 25-mL beaker to measure the ethanol. At least one of the glassware choices is inappropriate. Discuss which piece(s) of glassware is inappropriate, why it is inappropriate, and suggest a better choice.

First, I think we should use volumetric pipet for NaOH and ethanol instead of Erlenmeyer flask or 25-mL beaker because it could be the most precise and accurate glassware. Also, NaOH is an alkali substance so it could protect our hand with volumetric pipet. Second, I have to use 125.0 mL Erlenmeyer flask instead of 20mL because the total volume when combine substances together are greater than 35mL.

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