Lab 1: Accuracy in Measurement (Using the right tool for the Job)

and Density

Pre Lab Assignment: (must be completed before starting lab) Name, date, title of lab One to two sentences stating the objective of this lab Summary of lab procedure (brief paragraph in your own words) Answers to pre-lab Questions:

1. The 50 mL graduated cylinder is marked with mL incriments. How many decimal places should you record? Hint: you want to estimate one digit beyond the markings.

2. Is the estimated digit significant?3. Would you expect a piece of glassware marked to the nearest mL or

nearest tenth of a mL to be more accurate?4. Why is it best to determine the mass of the metal before placing it into

the graduated cylinder of water rather than after?5. What would happen if you used a very small amount of metal? How

would this affect your measurements? Hint: Consider the number of sig figs in your answer if you use more than 1.00 mL of metal or less than 1.00 ml of metal.

6. Why are specific gravity measurements unitless?

Part 1Introduction: When we work in a laboratory or medical environment it is important to make measurements that are accurate. Accuracy is related to the actual or “truthful’ value of the object measured. A ruler that can measure out to the millimeter is more accurate than a ruler that can measure out to the centimeter. Precision is how close a series of measurements with the same instrument are to each other, or how reproducible a measurement is.

How to determine accuracy? We will measure a volume of water with 3 different pieces of glassware each with varying degrees of accuracy and precision. By knowing the density of water (an exact number) and an accurate weight of a given volume of water we can ascertain the accuracy of the volume measured.

Objectives: - To understand the meaning of accuracy and precision—You will determine the

relative precision and accuracy of different glassware items.- Learn to measure volumes and masses (using volumetric glassware and top-

loading balances).

Notes:1. Measure water volume at the bottom of the meniscus.2. Use the same balance for the entire lab (do not move the balance!)

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3. Remember to use correct significant figures throughout when recording measurements and in calculations.

4. Perform all work at your station not at the balance. The balances are shared so do not monopolize them!

Procedure: Determination of Accuracy of various glassware items:1. Weigh a 10 mL syringe, a 10 mL graduated cylinder, and a 50 (or 100) mL

graduated cylinder separately and record the dry mass for each on your data sheet.

2. Using the markings on the side of each container to measure 10 mL of water with each glassware item. Do not use the 10 mL syringe to measure water for each container or transfer the same water from container to container. You want to measure the water using the marking on the container.

3. Weigh and record the mass for each item containing the 10 mL of water.

4. Record the temperature in the room on your data sheet and determine the density of water from the table provided.

5. Calculate the mass of the water & from that calculate the volume of water using the density recorded.

Part 2Introduction: By measuring the volume of water a solid displaces and by recording its weight we have the 2 components that make up density-- Mass and Volume.

Density = Mass of a sample (g)

Objectives: - Practice basic measurement techniques.- Determine the density of solid samples using water displacement method.

Notes:Record any aspects of the metal’s appearance that might help identify it.

Procedure: Determining the Density of Unknown Metals1. Choose a cube of unknown metal, weigh it and record the mass and letter.

2. Place enough water in the graduated cylinder that the metal cube will be completely submerged and record the volume. (The cube will fit into your 50 or 100 mL graduated cylinder)

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Volume (mL)

2

3. Tilt the graduated cylinder and add the metal cube letting it slide in rather than dropping.

4. Record the new volume .

5. Repeat steps 1-4 for 3 of the 5 unknown metals (be sure to record the letter for each metal studied).

6. For all 3 chosen metals calculate the volume of the metal and then the density of the metal and record.

7. Compare to the list of possible metals & their densities to identify the metal.

Part 3Introduction: The specific gravity of a fluid is a comparison of the density of a liquid sample to the density of water, as shown in the equation below. The density of water will vary with the Temperature, it is 1.00 g/mL at 4C. A chart will be provided in class to help you determine the density of water at the temperature the experiment will be conducted at.

Specific gravity = Density of liquid sample (g/mL)

A diagnostic tool in medicine is testing the specific gravity of a patients urine. A high specific gravity indicates that the urine is too concentrated. A few causes may be dehydration or excess glucose in the urine. Excessive glucose in the urine may be a result of poorly controlled diabetes. The normal specific gravity range for human urine is 1.002 to 1.030, anything above 1.030 indicates atypical waste products in the patients urine. In this lab you will be testing two unknown samples of fake urine to try to determine if the hypothetical diabetic patients need to adjust their diabetes control plans.

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Density of water (g/mL)

3

Notes: How to read a hydrometer: Place hydrometer in the liquid sample, spin slowly so

it does not stick to the sides of the container. To measure the specific gravity, read the marking on the hydrometer that the lowest point of the liquid’s meniscus falls on.

Procedure: Determining the Specific Gravity of Unknown Liquids1. Build a Hydrometer:

a. On a piece of take about 2.5 in long, use a ruler to draw markings every 1 cm, and ½ cm.

b. Cut the tip off of a plastic transfer pipetc. Wrap tape around neck of the pipet. d. Fill the pipet bulb about 2/3 full with sande. Plug the pipet opening with a small ball of clay.

2. Using the 50 or 100 mL graduated cylinder, place the hydrometer in 25 mL of water. Record the cm marking that the bottom of the liquid’s meniscus hits on the hydrometer.

3. Repeat step 2 using Sample A, which was prepared to have a specific gravity of 1.03, record the cm marking that the bottom of the liquid’s meniscus hits on the hydrometer. The results from step 2 & 3 will act as bench marks as you test unknown sample B.

4. Record the mass of an empty and dry 50 or 100 mL graduated cylinder.

5. Fill the graduated cylinder with about 25 mL of Unknown Sample B. Record the volume.

6. Record the mass of the graduated cylinder + Sample. (Subtract the mass recorded in step one from this number to get the mass of the water)

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7. Place a hydrometer in the liquid, spin it slowly so it does not stick to the sides of the graduated cylinder. Record the cm marking by reading the mark the bottom of the liquid’s meniscus hits on the hydrometer. Is this more or less dense then water and sample A?

8. Calculate the density of each Unknown sample B, and the specific gravity. Does it match the results of your hydrometer reading?

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Data & Observations WorksheetData 1A: Be sure to that all measurement include the correct number of significant figures and appropriate units.

Temperature _________________ Density of Water _____________________

100 mL grad. cyl. 10 mL grad. cyl. 10 mL syringeDry Mass

Dry Mass of container+ H2OMass of H2O(subtract the masses)Calculated Volume(from mass of H2O)

Observations:

Sample Calculations: (Show a sample of each type of calculation you did)

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Data 1B: Be sure to that all measurement include the correct number of significant figures and appropriate units.

Letter on Metal(A, B, C, D, or E)Physical AppearanceOf MetalMass of metal(g)

Initial Volumeof WaterVolume of water+ metal (mL)Volume of metal (mL)(subtract volumes)Density of metal(g/mL)Identity of metal

Observations:

Sample Calculations: (Show a sample of each type of calculation you did)

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Data 1C: Be sure to that all measurement include the correct number of significant figures and appropriate units.

Density of water (from chart) _______________________

Sample DI Water Sample A Sample B (unknown)

Mass of graduated cylinder (dry) X X

Mass of graduated cylinder + sample X X

Mass of sampleX X

Volume of sampleX X

Density of Sample (calculated)

X X

Specific Gravity (calculated) X X

cm Marking Measured with a hydrometer

Observations:

Sample Calculations: (Show each calculation)

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Post Lab Report

Results: o Please include Data/Observations Worksheets

Tables (1A, 1B, 1C) filled out with proper significant figures Legible observations & sample calculations

Error Analysis: o Part 2: Determine the percent error for the different metals. Discuss

the difference between your measured results and the given values by citing specific sources of error that may have occurred as you performed the lab (“human error” is not an acceptable answer).

Metal _______ Metal _______ Metal _______Percent Error

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Conclusionso Write a brief paragraph explaining what you would do differently

and what areas you can improve your lab technique. If you wouldn’t do anything differently please describe what lab techniques were important to identify the unknown metals and liquid.

Answers to Post-lab questions: Note that single word or phrase answers will not suffice. State the answer to the question followed by a brief description of the evidence supporting that answer.

1. Which Piece of Glassware (10 mL syringe, a 10 mL graduated cylinder or the 100/50 mL graduated cylinder) is most accurate according to your results? Explain why you came to this conclusion. Do your results match the expect results?

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2. Would the technique in part 2 work to measure materials with densities less than 1.00 g/mL? Explain why or why not.

3. Metals with greater atomic number often have higher density. Why do you think this is?

4. Healthy human urine has a specific gravity in the range of 1.002 and 1.030. Diabetic patients that do not have well controlled diabetes will have glucose in their urine that increases the specific density. Assume the unknown sample B is from a diabetic patient; diagnose if their current diabetes control plan is working.

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