02 Measurements in the Chemical Laboratory 2020 Spring PDF

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The Hess experiment could not have been easier. This helps make you understand the concepts, formulas and process....

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Principles of Chemistry I Laboratory

Montgomery College Rockville Campus

MEASUREMENTS IN THE CHEMICAL LABORATORY Introduction If you tell someone that you have 20 pennies, then the meaning is very clear. Pennies generally come in whole numbers and 20 pennies means twenty whole pennies. However, if you tell someone that you have a 20-gram bar of gold, the meaning is unclear. The actual amount of gold in the bar depends on how precisely it was measured, which in turn depends on the instrument used to make that measurement. Measured quantities are always written in a way that reflects the uncertainty in the measurement. If there is a large amount of uncertainty in the measurement, then the bar could be described as containing “20 grams of gold”. If a more precise balance was used, then the bar could be described as containing “20.0 grams of gold”. An even more precise measurement would be reported as “20.00 grams”. Significant figures are used to represent the precision of a measured quantity; the greater the number of significant figures, the greater the precision of the measurement. The number of significant figures in a written number can be determined using the following rules: 1. All nonzero digits are significant example: 5.196 (4 significant figures) 2. Interior zeros (zeros between two numbers) are significant example: 704 (3 significant figures) 3. Leading zeros (zeros to the left of the first nonzero digit) are not significant example: 0.00050123 (5 significant figures) 4. Trailing zeros (zeros at the end of a number) are only significant if there is a decimal point in the number or a bar () over the zero; example: 43.0 (3 significant figures) 10 (2 significant figures) otherwise the trailing zeros are not significant. example: 10200 (3 significant figures) Significant figures must also be taken into account when measured quantities are involved in calculations. The results of the calculations must reflect the precision of the initial measurements. When performing calculations involving measurements, be sure to follow the rules outlined below: Multiplication and Division – the result carries the same number of significant figures as the initial measurement with the fewest significant figures. example:

8.09 (3 sig. figs.)

x 21.435 x 0.30 = 52.022745 must be rounded to 52 (5 sig. figs.)

(2 sig. figs.)

(2 sig. figs.)

Addition and Subtraction – the result carries the same number of decimal places as the initial measurement with the fewest decimal places. example:

4.8 (1 decimal place) - 3.905 (3 decimal places) 0.895 must be rounded to 0.9

(1 decimal place)

Rounding – when rounding to the correct number of significant figures: Round down if the last digit dropped is 4 or less; Round up is the last digit dropped is 5 or more Measurements are written so that the uncertainty is in the last reported significant figure. For example, by reporting a mass of 20.0 grams, the individual means 20.0 ± 0.1 grams. By reporting a temperature change of 18°C, the individual means 18 ± 1°C. By reporting a time of 300 s, the individual means 300 ± 100 s. This experiment introduces some simple but important types of measurements commonly used by chemists. You will measure lengths of objects, masses of objects, and volumes of both liquid and solid samples using various instruments and techniques. It is important to remember that all measurements must be written with the proper number of significant figures and the appropriate units; a measurement without a unit is meaningless!

Pre-Lab Assignment Data Tables: Construct your data tables in your lab notebook using blue or black pen (no pencil) and a straightedge (ruler). Refer to the “Results” section to view the required format for your data tables. Your data tables must be neatly constructed in your lab notebook prior to lab. Pre-lab Questions: Some laboratory sections require online submission of prelab questions via Blackboard. Refer to your laboratory syllabus or check with your instructor if you are uncertain whether you are required to do so.

Experimental Procedure Throughout the experiment, be certain to take all measurements to the maximum precision allowed by your instrument. For example, the analytical balances should be set to record masses to ±0.001 g. When recording the volume of a liquid, read the measurement with your eye level with the bottom of the meniscus, and interpolate to one decimal place beyond the precision of the scale. For example, a 50-mL graduated cylinder with markings every 1 mL can be read to ±0.1 mL. Be certain to record all measurements to the correct number of significant figures, and include trailing zeroes when necessary. You will need to complete several tasks in this experiment. They can be completed in any order.

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Part 1a – Density of an Unknown Liquid using a Graduated Cylinder Using an analytical balance, determine the mass of a clean and dry 10-ml graduated cylinder. Add some unknown liquid to the graduated cylinder (at least 1 mL, but less than 10 mL). Record the volume of liquid to the correct number of significant figures, then determine the mass of the graduated cylinder with the liquid. Using the mass and volume of the liquid, you will calculate the density of the unknown liquid on the Post-Lab Worksheet. Part 1b – Density of an Unknown Liquid using a Buret Determine the density of the unknown liquid again. This time, determine the mass of a clean and dry 50mL beaker rather than a graduated cylinder. You will use a buret to measure the volume of unknown liquid. Before using the buret, be sure that the unknown liquid fills the tip (the entire region below the stopcock) and that there are no air bubbles anywhere in the tip. If there is not enough liquid in the buret, refill it from the stock bottle before you begin dispensing the liquid. Record the initial volume of liquid in the buret, remembering to read from the bottom of the meniscus and interpolate between the lines of the scale. Dispense 15-20 mL of the unknown liquid from the buret using the stopcock on the bottom, then record the final volume in the buret. The volume of liquid dispensed will be the difference between the two measurements. Determine the mass of the beaker with the unknown liquid. Part 2a – Density of Cylindrical Solid using Water Displacement Measure the mass of the cylindrical solid and then determine the volume of the solid using the method of water displacement. Add some water to a 50-mL graduated cylinder and record the volume. Add the cylindrical solid to the graduated cylinder, then record the volume again. If the final volume is greater than 50 mL, or if the cylindrical solid is not completely submerged in water, repeat the measurement with a different initial volume of water. Part 2b – Density of Cylindrical Solid using Direct Measurements Use the same cylindrical solid. Measure the mass (again). Use Vernier calipers to measure the diameter and length of the solid. To use calipers, place the object in the lower portion of the large jaws of the calipers:

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To read the calipers, use the bottom scale (in metric units, rather than inches), noting that there is a fixed scale and a sliding scale. The sliding scale has several tick marks. The left-most of these, labeled “0”, marks the whole number of millimeters when measured on the fixed scale. The example to the left measures 21 mm, since the leftmost sliding scale tick mark is between 21 mm and 22 mm on the fixed scale.

20

30

40

To determine the decimal place, look for where the tick marks on the sliding scale align with the tick marks on the fixed scale. In the example, this alignment occurs at the “3” of the sliding scale. Thus, the measurement is 21.3 mm. Note that the calipers in the lab may have more tick marks on the sliding scale, so you may be able to take measurements to a greater precision than in the example. It may be difficult to determine where the alignment occurs, so the last digit may be read differently by different people. Regardless of the value of the last digit, it is mathematically significant. After you measure the diameter and length of the cylindrical solid, you can calculate its volume. Part 3 – Thickness of a Metal Foil Select a piece of metal foil and record the identity of the chosen metal. Look up the density of your metal in the CRC Handbook of Chemistry and Physics. You will find this information in the section entitled “Physical Constants of Elements and Inorganic Compounds”. Measure the mass of the metal foil and then use the calipers to measure its length and width (not the thickness). Instructions on how to read the Vernier scale can be found in Part 2b. Use your measurements to calculate the thickness, in cm, of the metal foil. Be certain to cite your source for the density of the metal.

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Results Be sure the balance is set to ±0.001 g. Record all measurements in your lab notebook in blue or black pen using the format listed below. Make sure that all of your measurements contain the appropriate units and the correct number of significant figures. Turn in the top perforated original data sheet from your lab notebook before leaving lab. Part 1a – Density of Unknown Liquid using Graduated Cylinder Mass of empty graduated cylinder Mass of graduated cylinder + unknown liquid Volume of unknown liquid Part 1b – Density of Unknown Liquid using Buret Mass of empty 50-mL beaker Initial liquid level in buret Final liquid level in buret Mass of beaker + unknown liquid Part 2a – Density of Cylindrical Solid using Water Displacement Mass of solid Initial volume of water in graduated cylinder Final volume of water in graduated cylinder Part 2b – Density of Cylindrical Solid using Direct Measurements Mass of solid Diameter of solid Length of solid Part 3 – Thickness of a Metal Foil Identity of metal foil Density of metal* Mass of metal foil Width of the metal foil Length of the metal foil *use the CRC Handbook of Chemistry and Physics (available during lab) to find this value. Post-Lab Worksheet The Post-Lab Worksheet must be completed and turned in, prior to the end of lab. You can complete this worksheet using pen or pencil. 5

MEASUREMENTS IN THE CHEMICAL LABORATORY Grade Sheet

Results (due at the end of lab) Data tables were constructed in lab notebook prior to lab Data recorded in lab notebook in blue or black ink Includes correct units and significant figures Post-Lab Worksheet (due at the end of lab) Units are shown in set-up and answer Proper significant figures are shown in set-up and answer Complete work is shown for all calculations Calculations include correct units and significant figures Questions are answered correctly All work is organized and legible Student Comportment On time (not 1-5 minutes late), remembers drawer number and lock combination Prepared, understands procedure, works efficiently and independently Works with high regard to personal safety and the safety of others Respects, maintains, and cares for lab materials and equipment; limit breakage Thoroughly cleans personal and fair share of common glassware, materials, and areas of lab Completes experimentation and clean-up by the end of lab

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Name: _________________________ Date the experiment was conducted: _____________ MEASUREMENTS IN THE CHEMICAL LABORATORY

POST-LAB WORKSHEET

All calculations must be neatly written and must contain the appropriate units and the correct number of significant figures. Use dimensional analysis when converting. In order to receive credit, your work must be submitted on this worksheet (not other paper) and be legible. This Post-Lab Worksheet is due at the end of lab. CALCULATIONS Part 1a – Density of Unknown Liquid using a Graduated Cylinder Mass of unknown liquid

Density of unknown liquid

Part 1b – Density of Unknown Liquid using a Buret

Mass of unknown liquid

Volume of unknown liquid

Density of unknown liquid

Part 2a – Density of Cylindrical Solid using Water Displacement

Volume of cylindrical solid

Density of cylindrical solid (present final answer in g/mL)

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Part 2b – Density of Cylindrical Solid using Direct Measurements Volume of cylindrical solid in cm3 (note: Vcyl = πr 2h)

Density of cylindrical solid (present final answer in g/cm3 )

Part 3 – Thickness of a Metal Foil Surface Area of one large side of the metal foil in cm2

Volume of metal foil (from mass measurement and literature density)

Thickness of metal foil in cm

QUESTIONS 1. Do the volume measurements of Part 1a and Part 1b have the same precision? Explain why or why not. Which measurements has the greater precision and why?

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2. Compare and contrast procedures of Part 2a and Part 2b in terms of a) ease of measurement (consider both time and labor)

b) precision of the results (explain your answer)

3. a. Convert the thickness of your metal foil from Part 3 into nanometers (nm). Show your work using dimensional analysis in one long set-up and the conversion factor 1 nm = 10-9 m or 109nm = 1 m. No discrete calculations are permitted. Remember to pass through the base unit of meters.

b. Convert the thickness of your metal foil into Ångstroms (Å). Show your work using dimensional analysis in one long set-up and the conversion factor 1 Å = 10-10 m or 1010 Å = 1 m. No discrete calculations are permitted. Remember to pass through the base unit of meters.

4. Explain why it is not possible to use the water displacement method utilized in today’s experiment to determine the volume of the metal foil in Part 3.

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