Weighing measuring accuracy and precision lab report

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How is Lab Glassware Used? Introduction: In general chemistry lab, you will come into contact with common laboratory glassware, such as, beakers, Erlenmeyer Flasks, volumetric flasks, pipettes, burets, and graduated cylinders. Beakers and Erlenmeyer flasks are usually to hold liquids throughout an investigation. When you look closer at each glassware, you’ll notice markings on each measure specific volumes. Glassware is generally divided into two types, those that hold certain volumes and those that deliver certain volumes. The purpose of this investigation is to find the most precise and/or accurate glassware based on the average lowest % error. Accuracy is the how close a measured value is to the actual (true) value. While, precision is the repetition of measurements under unchanged conditions and shows the same results. The more precise and accurate a measurement is, the less error with density calculations expressed, using significant figures. Density (g/mL or g/cm3) is the substance is its mass (grams) per unit volume (mL or cm3), in other words,

𝐷𝑒𝑛𝑠𝑖𝑡𝑦 = 𝑚𝑎𝑠𝑠

𝑣𝑜𝑙𝑢𝑚𝑒

Significant figures are used as the number of digits that carry real information about a measurement. There is no such thing as a perfect measurement. Which is why usually in an experiment, repeated trials are recorded to ensure verifiable data. The objective is to determine the relationship between percent error, and measurement accuracy throughout the experiment. Every lab equipment has some degree of uncertainty so you can estimate one more digit past the smallest division on the measuring device. For example, for a 10mL graduated cylinder the smallest graduation is tenth of a milliliter (0.1mL). That means when you read the volume, you can estimate to the hundredths place (0.01mL). However, 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. In the case of the glassware used in general chemistry lab, both the 10mL volumetric pipette and 50mL volumetric flask will have two sig figs after the decimal point (i.e. 10.00mL and 50.00mL). For the 150mL beaker and the kitchen measuring cup, assume that 50.mL has two sig figs (it will not be obvious based on the volume markings).

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Procedure for Part 1: Determining Precision and

Accuracy of Common Glassware

Objectives:

 Determine how to use the balance to find the mass of water in certain glassware.

 Determine proper procedures to measure volume of water and report data in correct number of significant figures.

 Determine the relationship between percent error, and measurement accuracy throughout the experiment.

 Determine the number significant figures in calculations using measurements from investigation.

Before you come to lab, in your notebook create a data table for each glassware

used that includes 4 rows (headings and trial numbers) and the following

columns:

 Mass of glassware with a certain volume of water (g)  Mass of water (g)  Volume of water (mL)  Experimental value of density (g/mL)  % error

You should also include an area to record the temperature of the water used throughout

the investigation, the density (g/mL) of water at this temperature (Theoretical Value),

empty glassware (g), average density (g/mL), and average % error.

Throughout the investigation a 50 mL beaker, 125 mL Erlenmeyer flask, 50 mL buret, 10 mL graduated cylinder, thermometer, plastic pipette, weighing boat, goggles, and an electronic balance will be used. When recording mass, all numbers should be recorded and never rounded. Also, the appropriate number of significant figures (will vary depending on glassware) should be used when recording volumes of each glassware. In your proposal address the following and record everything in your notebook:

How many trials will you run?

 How will you and your group divide the work?

Obtain approximately three hundred milliliters of distilled water in a 600 mL beaker. This water will be used for the entire investigation.

 Should you take the temperature of the water? Why or why not is this important?

 How long should you leave the thermometer in the water?

 Should you record the temperature in your notebook?

 Where in your lab manual can you find the theoretical values for the density of water at a certain temperature?

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Mass an empty dry 50 mL beaker.

 How much distilled water from the 300mL stock of water should you place in the 50 mL beaker?

 What should you use to add the water drop-wise until the bottom of the meniscus corresponds to a specific volume? If any water was spilled during the transfer, the beaker was wiped with a paper towel.

 Should you take the mass of the 50mL beaker with water in it? Why or why not? The water from the glassware should be put back into the 600 mL beaker to use for the rest of the investigation.

 How many more times should you repeat the steps above for the 50 mL beaker?

 Can you use the empty mass for the 50 mL beaker for all the trials?

Repeat the above procedure with a 125 mL Erlenmeyer flask and same electronic

balance.

Procedure for Part 2: Determining Precision and

Accuracy of A 50 mL BURET

Objectives:

 Determine how precise and accurate a 50 mL buret is. Before you come to lab, in your notebook create a data table (if you didn’t do so

in Part 1) that includes 4 rows (headings and trial numbers) and the following

columns:

 Mass of glassware with a certain volume of water (g)  Mass of water (g)  Volume of water (mL)  Experimental value of density (g/mL)  % error

You will be following a similar procedure as used for Part 1.

 Do you need to take the mass of the empty dry 50mL buret? Can you do that on the electronic balances provided?

Set-up buret using a ring stand and clamps to add distilled water to it.

 How much distilled water from the 300mL stock of water should you place in the 50 mL buret?

Mass an empty dry 100 mL beaker using the same electronic balance as in Part 1.

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 Using the stopcock, how much volume of distilled water should be delivered into the empty, dry 100 mL beaker from the buret? All of it?

 Should you take the mass of the 100 mL beaker with water in it? Why or why not? The water from the glassware should be put back into the 600 mL beaker to use for the rest of the investigation.

 How many more times should you repeat the steps above for the 50 mL buret?

 Can you use the empty mass for the 100 mL beaker for all the trials?

Procedure for Part 3: Determining Precision and

Accuracy of A 10 ml Graduated cylinder

Objectives:

 Determine how precise and accurate a 10 mL graduated cylinder is. Before you come to lab, in your notebook create a data table that includes 4 rows

(headings and 3 trials) and the following columns:

 Mass of glassware with a certain # of drops to reach 1mL (g)  Mass of water (g)  Volume of water (mL)  Experimental value of density (g/mL)  % error

You will be following a similar procedure as used for Part 1.

Mass an empty dry 10 mL graduated cylinder using the same electronic balance as in Part 1 and 2.

 How many drops are in 1 mL of distilled water?

 What will you use to add drops into the 10 mL graduated cylinder?

 Should you take the mass of the 10 mL graduated cylinder with water in it? Why or why not? How many more times should you repeat the steps above for the 10 mL graduated cylinder?

 Can you use the empty mass for the 10 mL graduated cylinder for all the trials?

Divide the mass of 1 mL of distilled water by 1 mL (volume) to find the density.

Procedure for Part 4: Determining THE mass of

sand

Objectives:

 Develop a procedure for weighing out exactly 1 gram of sand by using the “TARE” button on the balance.

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Before you come to lab, in your notebook create a procedure and an area to place

your results for Part 4.

Place about 3 scoops of sand into a 50 mL beaker using your scoopula. The sand is located in the fume hood and should be treated as if it were a chemical. Remember to take an approximate sample out of the chemical container and put it in a weighing boat or on weighing paper. To avoid contamination of the chemicals, do not put your scoopula directly into the original chemical reagent container, or put excess chemical back into the original chemical reagent container.

 What will you take the mass of first? Remember, sand (or any solids) can’t go directly on the balance.

 What button should you press on the balance to “zero” it out? What mass does this get rid of?

Take the weighing boat off the balance and place a small amount of sand into the

weighing boat using the scoopula. Place the weighing boat with sand back on

the balance.

 Did you take the mass of exactly 1 gram of sand?

If you didn’t take the mass of exactly 1 gram of sand, add or remove accordingly

using your scoopula until the desired mass is reached. The weighing boat

should be taken off the balance every time solid is added or removed.

 Will you record all the numbers shown on the balance, or should you round? Record the mass of sand in your lab notebook with the proper units.

 How will you dispose of the used and unused sand? Any solids taken out of the reagent bottle (used or unused) should NEVER be put back.

 Why is using the “TARE” button useful?

 How would you measure the sand if you forgot to “TARE” your weighing boat first?

 Now measure exactly 1 gram of sand without using the “TARE” function. Calculate the mass of the sand only. What other measurement do you need?

Clean up and Waste Collection

 When you have finished your experiment, place all distilled water down the sink and return glassware to your drawer(s).

 DO NOT place sand back in the original container, put sand in the designated SOLID WASTE container.

 Clean up your bench area and balance room.

Lab Report Guidelines General (points can be deducted if these items are not followed)

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o Title page: TA’s name, Date, Section, & Group member names o Written in third person and past tense o Proper grammar and spelling o Double-spaced o Label each of the sections (listed below) o Attached Team Contribution Forms from each group member

Intro (0.5 pt):

o Theory: Describe important concepts. What is the difference between accuracy and precision?

o Discuss the importance of significant figures. o Discuss the concept of density. State and explain the formula. o Brief description of the goal in this experiment/Problem to solve?

Procedures (0.5 pt):

o In paragraph format, describe:

 The equipment, glassware, containers with volumes (mL), any equipment used.

 The actual procedures. **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):

o Include temperature of Water (◦C) used throughout investigation. o Include density (g/mL) of Water at this Temperature (Theoretical Value) o USE APPROPRIATE SIGNIFICANT FIGURES IN EACH TABLE! Also don’t

forget to name and briefly describe all your tables. o Include tables for the glassware measurements, including mass, volume,

measured (experimental) density, % error, average experimental densities, and average percent error, mass of sand both with and without using the “TARE” function. When creating your tables, use the following as an example:

Table 1: Average density of 50 mL beaker filled with 50 mL of distilled water.

Mass of EMPTY DRY 50 mL Beaker (g): _________

Trial Mass of 50mL Beaker

with water (g)

Mass of

Water (g)

Volume of Water (mL)

Experimental Value of Density (g/mL)

% Error

1

2

3

Average Density (g/mL): _________ Average % Error: ___________

o Include a table for the volume and mass of your one drop of water

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o Show 1 example using your actual numbers for all calculation performed, along with units (mass of water, density, average density, % error, and average % error). When you add or subtract, report answer in least # of decimal places. When you multiply or divide, report answer in least # of significant figures.

o Indicate where the data came from. Equation Editor on MS Word is helpful. o USE APPROPRIATE SIGNIFICANT FIGURES! o When reporting calculations, use the following as an example:

Sample Calculations: Using data from Table # ____,Trial # ____. Mass of glassware WITH Water --- Mass of EMPTY DRY glassware = Mass of Water (g)

Discussion (0.5 pts):

o Briefly restate the purpose of the experiment. o 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.

o Discuss the significance of significant figures and precision/accuracy. o Discuss your sources of error. “Human error” is not acceptable as a source of

error (e.g. John Smith was late for lab so the group didn’t have anyone to record investigation data). Acceptable “human error” as a source of error include, (e.g. Spilling water, or forgetting to take the temperature of the water). a) Why did you have % error in your densities (human error doesn’t count)? b) Why did your volume measurements vary so much (human error doesn’t

count)? c) What could you have done differently to improve your results (human error

doesn’t count)? d) If you were asked to measure 38 mL of water, which lab equipment (based on

today’s lab) would you use and why? e) If you were asked to measure 38.50 mL of water, which lab equipment (based

on today’s lab) would you use and why?

Conclusion (0.5 pt): o Wrap up. Did you meet the goals of the lab? o Report your results to the company. Support your recommendation to the

company.

Works Cited (0.5pt)

o Reference the lab manual and any other reputable sources using ACS format.