How is Lab Glassware Used For Students 101 revised F19 PDF

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What types of glassware to use when measuring certain things...

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How is Lab Glassware Used? BACKGROUND The glassware and equipment/apparatus used in our general chemistry labs to measure a quantity has inherent limitations. There are also differences in how people make the same measurement. Uncertainties always exist in measured quantities and we use the terms accuracy and precision to better understand them. Accuracy is how close a measured value is to the actual (true) value. The degree of uncertainty in any measured value will depend on the apparatus or glassware used. Precision is measured by repeating measurements under unchanged conditions and comparing how widely the results vary. The precision of the measurements may be expressed qualitatively or by calculations in terms of the standard deviation. When we perform a measurement in the laboratory, we should remember that there is always some uncertainty in the last digit reported for that measured quantity. All digits of a measured quantity, including the uncertain one, are called significant figures. A more precise measuring device allows for measurements to be made with more significant figures. The precision of an experimental result is no more precise than the measuring device used with the least precision.

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Percent Error A percent error calculation tells us about the accuracy of an experimental measurement (or an experimental result).

% 𝐸𝑟𝑟𝑜𝑟 =

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|𝐸𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡𝑎𝑙 𝑉𝑎𝑙𝑢𝑒 − 𝐴𝑐𝑐𝑒𝑝𝑡𝑒𝑑 𝑉𝑎𝑙𝑢𝑒| × 100 𝐴𝑐𝑐𝑒𝑝𝑡𝑒𝑑 𝑉𝑎𝑙𝑢𝑒 School of Molecular Sciences

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Performing Measurements Using Common Glassware Lab equipment used to measure volumes varies in size, types, and purpose. Some common glassware used in lab include: beakers, burets, graduated pipets, graduated cylinders, and volumetric flask.

Figure 1. Some common glassware equipment used in lab. Each of these pieces of glassware are commonly used in the laboratory. Each type of glassware is used to either hold or deliver certain volumes of liquids, but they vary in the amounts and accuracy with which they can measure these volumes. The glassware used to measure volumes has some degree of uncertainty, therefore, when reading a measurement you should estimate one more digit past the smallest division on the measuring device being used. For example, a buret or a 10mL graduated cylinder has gradations of a tenth of a milliliter (0.1 mL), therefore, volumes should be estimated to the hundredths place (0.01 mL). When measuring out a specific volume using graduated glassware, be sure to line up the desired volume line with the bottom of the meniscus.

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Reading the liquid volume in this 50 mL graduated cylinder with gradations that are to the nearest mL, we would measure the volume to the nearest tenth of a mL at the bottom of the meniscus, so the correct reading would be 43.0 mL.

Figure 2. Graduated cylinder reading Some glassware such as volumetric flasks and volumetric pipettes only have a single line to indicate volume. This is because they are made to measure just one specific volume and will have two significant figures after the decimal point (i.e. 10.00 mL and 50.00 mL). Alternately, for most beakers with gradations every 10 or 20 mL, a volume measured to the line can only be assumed to have 2 significant figures (i.e. 50. mL). This may not be obvious based on the volume markings. Performing Measurements Using an Electronic Balance Balances also have some degree of uncertainty. An electronic balance should be properly calibrated so that the masses measured are accurate. When recording a measured mass, all numbers displayed should be recorded and never rounded. For example, if the display is showing 24.907 g, then all five numbers are significant and we should record the exact number displayed (24.907 g).

Figure 3. Electronic Balance

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The recommendations below will help you when performing measurements using a balance: ● “Tare” or “Re-Zero” the balance before each measurement. Press the “Tare” or “Re-Zero” button after placing your weighing boat or glassware on the balance, but before you start measuring the mass of the chemical reagent. Be sure the display reads all zeros (for example, 000.000 g). ● Carefully transfer reagent to the weighing container. o Make sure that reagents being weighed do not spill on the balance pan. o For some corrosive or messy reagents, it is best to remove the weighing container before adding and removing reagent, and placing it back on the balance for measurement. ● Monitor any fluctuation on the display. Minor fluctuations may be observed on the value displayed, especially if there is a draft. Fluctuations may be minimized by using a balance shield. o Always record all of the values displayed. If there is a drift, record your best guess of all digits. ● Record all values from the display. When recording mass, all numbers should be recorded and never rounded. All values on display are significant! ● Never place chemicals directly on balance pan. Select a clean and dry weighing boat or glassware in which to place the solid reagent, and place it on the balance. Be sure to record its mass and/or tare the balance before measuring the chemicals.

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INVESTIGATION GOAL The purpose of this investigation is to develop familiarity with measuring liquid volumes using common laboratory glassware, understanding how to accurately measure liquids and solids on a balance, and using the data collected to determine which piece of glassware gives you the most accurate and precise measurements. In Part 1, you will perform/develop an experiment to determine the precision and accuracy of each of the following glassware: a beaker, a graduated pipette, a buret, and a graduated cylinder. In Part 2, you will determine the volume and mass of a drop of water, and develop a conversion factor for the number of drops it takes to make a 1.0 mL volume. In Part 3, you will develop a procedure for weighing out various amounts of sand using an electronic balance.

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Procedure for Part 1: Determining Precision and Accuracy of Common Glassware

I.

OBJECTIVES ● Determine proper procedures to measure the volume of water with four different types of glassware, and report the data with the correct number of significant figures. ● Learn how to determine the proper laboratory equipment to use based on what you are trying to measure, and how precise the measurement needs to be. ● Determine the correct number of significant figures to use and report when performing calculations using measurements obtained during this part of the investigation.

II.

PROCEDURE DEVELOPMENT

Develop a procedure to measure a specific volume of distilled water with each of the following pieces of glassware: a 50 mL beaker, a 10 mL graduated cylinder, a 50 mL buret, and a 10 mL graduated pipet. Important things to consider when writing your procedure: Beakers and graduated cylinders hold a specific volume and pipets and burets deliver a specific volume. ● Decide on how much distilled water you will measure for each piece of glassware. This volume will only serve as a guide. Once you have performed the experiment, be sure to record in your lab notebook how much distilled water you actually measure for each piece of glassware. o Proposed volumes for: 50 mL beaker: ______ 10 mL graduated cylinder: ______ 50 mL buret: ______ 10 mL graduated pipet: ______ ● Develop a method of accurately weighing the measured/dispensed volumes from each piece of glassware so that a % error in mass can be determined for each. ● Ensure that the bottom of the meniscus reaches the specific volume. You can achieve this by using something that will allow you to add the water drop-wise. ● Note that the markings on a buret stop before they reach the bottom. This means measured volumes using burets must start at a known marking and end at another marking for accurate measurements. ● Note that pipets have markings that seem to include or start from the tip of the tube. This means pipets are meant to be fully dispensed after filling to a measured marking.

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III.

EXPERIMENTAL CONSIDERATIONS

In order reduce waste, we suggest that you obtain approximately 300 mL of distilled water in a 600 mL beaker to be used for the entire investigation. The water from the glassware should be put back into the 600 mL beaker after each measurement to use for the other parts of the investigation. The appropriate number of significant figures should be used when recording volumes using each type of glassware. This will vary depending on glassware used. Specific Waste & Safety: All water used in this part can go down the sink. IV.

DATA COLLECTION & CALCULATIONS

It is important to keep a record of all measurements in your lab notebook. A table for organizing the data can be helpful. For this lab we recommend that you record the following data for each piece of glassware used. ❑ Name of glassware used ❑ Experimentally measured volume (mL) ❑ Calculated mass of water (g) ❑ Experimentally measured mass of water (g) ❑ % Error and average % Error for glassware For this lab we have provided an example of what the data and calculations table might look in your notebook. Distilled water has a Density of:___________ g/mL at a Temperature of: ______oC, Table 1-4: Measurements for a _________________ Experimentally Measured Glassware Trial Volume (mL) 1 2

Calculated Mass of Water (g)

Experimentally Measured Mass of Water (g)

% Difference in Mass Measurements

Average % Difference: ___________%

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Using Density to Calculate Mass In this part of the investigation you will need to calculate the mass of your water samples from the volume measurements and the known density values. The following link: http://jupiter.plymouth.edu/~jsduncan/courses/2012_Spring/Techniques/Exams/Density OfWater-vs-Temp.pdf contains a table of densities of water at various temperatures. Follow correct significant figure rules to determine how many 𝑀𝑎𝑠𝑠 significant figures should be reported in your calculations. 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 = 𝑉𝑜𝑙𝑢𝑚𝑒 Calculate the % difference between two values: Because both of these masses are experimentally derived, we can use a percent difference equation to compare how much the two mass values deviate. % 𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒 =

|𝑉𝑎𝑙𝑢𝑒 1 − 𝑉𝑎𝑙𝑢𝑒 2|

𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑜𝑓 𝑉𝑎𝑙𝑢𝑒 1 𝑎𝑛𝑑 2

× 100

Procedure for Part 2: Determining volume and mass of a drop of water

I.

OBJECTIVES ● Determine the volume and mass of a drop of water. ● Develop a conversion factor for the number of drops it takes to make 1.0 mL volume.

II.

PROCEDURE DEVELOPMENT

Develop and write a procedure for determining the volume and mass of a drop of water. Important things to consider when writing your procedure: ● Decide on the number of drops you will use to be able to get acceptable average values of mass and volume. We recommend anywhere between 20 and 100 drops. ● Decide which piece of glassware you will use. Be sure to record its mass. ● Decide on the number of trials you will use for this part of the experiment. ● Calculate a ratio of the number of drops used and the volume in mL, drops/mL. III.

EXPERIMENTAL CONSIDERATIONS ● It is important to stay consistent with the person that is transferring the water into the glassware. ● It is important to obtain the mass of each of the glassware pieces when they are empty and dry as well as their mass with water in them. ● It is important to stay consistent with the balance you decide to use.

Specific Waste & Safety: All water used in this part can go down the sink.

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IV.

DATA COLLECTION & CALCULATIONS

For this part of the investigation we recommend that you record the following data and calculations. ❑ Experimentally measured number of drops ❑ Mass of dry container ❑ Mass of container with water ❑ Mass of one drop of water Table 5: Measurements for an average volume and mass of one drop of water. Container used:________________________ Trial #

Experimentally Measured Number of Drops

Mass of Mass of Full Mass of Empty Dry Container One drop Container with Water of Water (g) (g) (g)

Calculated Volume of one drop (mL)

Calculated number of drops/mL (drops/mL)

1 2 Averages:

Procedure for Part 3: Measuring Solids with an electronic balance

I.

OBJECTIVES ● Learn how to correctly use an electronic balance to weigh masses. ● Learn how and why to use the “TARE” or “Re -Zero” function on an electronic balance. ● Be able to calculate amount of mass loss, if any, when transferring a solid to a reaction container.

II.

PROCEDURE DEVELOPMENT

Develop and write a procedure to exactly measure 0.100 g, 1.000 g, and 10.000 g of sand by using a secondary container. You should be able to determine the amount of sand lost, if any, when you transfer each of your measured quantities to a reaction container. Important things to consider when writing your procedure: ● Collect an appropriate amount of solid in a secondary container to bring to the balance room. o Ex: Place about 3 scoops of sand into a 50 mL beaker with a clean, dry scoopula. ● You should never weigh the solid directly on the balance pan, you can use a weighing boat or something such as a watch glass or clean, dry glass beaker of sufficient size. o Decide what you will use. Arizona State University

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● Read the background section “Performing Measurements Using an Electronic Balance” to become familiar with how to use and tare/zero the balance when measuring mass. ● Determine the best way to add and remove excess sand when performing your measurements. ● Keep a record of all of your measurements with the correct number of significant figures. III.

EXPERIMENTAL CONSIDERATIONS

How to handle solid reagents from a reagent bottle. ● The sand is located in the fume hood and should be treated as if it were a chemical reagent. ● The stock chemical container should never leave the hood. An approximate amount of material should be dispensed in the hood to a secondary container to bring with you to the balance. o Use the spatula or scoopula provided to dispense the solid into your secondary container to avoid contaminating the stock reagent. o Use a clean and dry beaker or weighing boat as your secondary container to bring to the balance room. ● Never return reagents back to the original stock chemical container. It is considered contaminated once removed. ● If there is any unused solid after measurement, dispose of it in the appropriate waste container. Waste containers for each lab can be found in the fume hood and are labeled for your specific experiment. Specific Waste & Safety: All water used in this part can go down the sink. Dispose of sand in the designated SOLID WASTE container. IV.

DATA COLLECTION & CALCULATIONS

For this part of the investigation we recommend that you record the following data and calculations (see previous sections for suggested data table layouts). ❑ Record the procedure you followed to record the thee suggested masses of sand along with any of your observations an potential sources of error. FINAL CLEAN UP AND WASTE COLLECTION When you have finished the entire experiment return glassware to your drawer(s). All distilled water used in this investigation can go down the sink. Solutions containing dye should go in the designated WASTE container in fume hood. DO NOT place sand back in the original container, dispose of sand in the designated SOLID WASTE container in the fume hood. ● Clean up your bench area and balance room. ● ● ● ●

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LAB REPORT GUIDELINES General (points can be deducted if these items are not followed)  Title page: TA’s name, Date, Section, & Group member names  Written in third person and past tense  Proper grammar and spelling  Double-spaced  Label each of the sections (listed below)  Attached Team Contribution Forms from each group member Introduction (1pt):  Theory: Describe important concepts. What is the difference between accuracy and precision?  Discuss the importance of significant figures.  Discuss the concept of density. State and explain the formula.  Brief description of the goal in this experiment/problem to solve? Procedures (1pt):  In paragraph format, describe the equipment, glassware, containers with volumes (mL), etc. for all parts of the procedure.  **PAST TENSE!!** Be concise and descriptive. This should be detailed enough that another group could duplicate your work. Write as paragraphs, not bullets. Results & Calculations (5.5 pts):  Include temperature of Water (◦C) used throughout investigation.  Include density (g/mL) of Water at this Temperature (Theoretical Value)  USE APPROPRIATE SIGNIFICANT FIGURES IN EACH TABLE! Also don’t forget to name and briefly describe all your tables.  Include tables for the glassware measurements, the volume and mass of one drop of water measurements, and the sand mass measurements.  Show sample calculations: o Show only one example calculation for each type of calculation performed using your actual numbers. o Indicate which table and trial the numbers came from. o Include units on all numbers. o When you add or subtract, report answer in least # of decimal places. o When you multiply or divide, report answer in least # of significant figures. o Equation Editor on MS Word is helpful. o USE APPROPRIATE SIGNIFICANT FIGURES! o When reporting calculations, use the following as an example:

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Sample Calculations: Using data from Table # ____,Trial # ____. Mass of glassware WITH Water - Mass of EMPTY DRY glassware = Mass of Water (g) Discussion (1pt):  Briefly restate the purpose of the experiment.  Rank each of the measuring devices. Which one did you find to be most accurate/precise based off your experiments? Discuss how you were able to determine this ranking.  What relationship did you observe between the volume of water and the mass of water in part 1? Could this relationship be used as a conversion factor for mL to g?  Discuss your sources of error. Be specific. Be careful not to use a blanket “Human error” excuse. (Unacceptable Example: “John Smith was late for lab so the group didn’t have anyone to record investigation data).” Acceptable example: “The water was spilled on the balance pan.” or “The temperature was estimated based on another group’s data because it wasn’t recorded.” etc. ) o If your volume measurements varied widely, what might be the cause? (human error doesn’t count)? o What could you have done differently to improve your results (human error doesn’t count)? o If you were asked to measure 38 mL of water, which piece of lab glassware (based on today’s lab) would you use and why? o If you were asked to measure 38.50 mL of water, which piece of lab glassware (based on today’s lab) would you use and why? Conclusion (1pt)...