Chemistry 101

 

Lab 1: Measurement and Density

Objectives:

a. Determine the relative precision and accuracy of different glassware items.

b. Determine the density of a solid sample by two methods

Skills:

            -Practice basic measurement techniques of masses and volumes

            -Review accuracy and precision in measurement

 

Pre-lab questions:

Read both parts of the procedure; then answer these questions on a separate sheet of paper before coming to class on Thursday; turn them in at the beginning of class. This is an assignment that may be done by each lab partnership (in other words, one sheet for each lab partnership).

 

1. The 10 mL graduated cylinder is marked to tenths of a mL. How many decimal places should you record (how many are significant)?

 

2. Why is it best to determine the mass of the metal before placing it into the graduated cylinder of water rather than after?


3. What would happen to your accuracy if you used a very small amount of metal? How would this affect your measurements?

 

Materials needed:

            ¥ Prepared graph paper (see prelab)        ¥ 10 mL graduated cylinder

            ¥ 100 mL beaker                              ¥ 50 or 100 mL (ÒLargeÓ) graduated cylinder

            ¥ small beaker                                  ¥ 10 mL volumetric pipet and suction device

            ¥ distilled water (not tap water)                ¥ electronic balance

            ¥ thermometer                                             ¥ calipers or rulers

            ¥ two different metal cylinders

            ¥ data sheet (print this out as a separate document)

 

Remember:   Measure water volume at the bottom of the meniscus.

                        Use the same balance for the entire lab.

 

Procedure (work in pairs, but each person should turn in their own data sheet):

 

Part One — the density of water

 

1. Weigh a 100 mL beaker, a 10 mL graduated cylinder and a 50 or 100 mL graduated cylinder and record the dry mass for each on the data sheet.

 

2. As best as you can, put 10 mL of distilled water in each glassware item. Do not use the 10 mL graduated cylinder 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. Record the volume for each one, remembering that you should estimate one place past the markings. DonÕt forget trailing zeroes.

 

3. Weigh and record the mass of the glassware plus water for each item.

 

4. Weigh and record the mass of the small beaker. As best as you can, draw 10 mL of distilled water into the 10 mL volumetric pipet with the suction device and then expel that volume into the small beaker. Re-measure and record the mass of the beaker and water.

 

5. Measure the water temperature. Consult a reference book, such as the CRC Handbook, or a reference site, such as the Frostburg (MD) State University chemistry site http://antoine.frostburg.edu/chem/senese/javascript/water-density.html to determine the density of water at your observed temperature.

 

Part Two — the density of a metal

 

6. Weigh and record the mass and letter of a metal cylinder. The metal pieces should fit into your 10 mL graduated cylinder and have a volume between 1.00 mL and 3.00 mL

 

7. Measure and record the diameter and height of the cylinder.

 

8. Place enough water in the 100 mL graduated cylinder so the water has about the same height as the metal cylinder. Record the water volume. Tilt the graduated cylinder and gently slide the metal cylinder down inside it. Record the volume of water plus metal cylinder. The difference between these two measures is the volume of the metal cylinder. This technique is called water displacement. Remove and dry the metal cylinder and return it to the lab cart.

 

9. Find out the identity of the metal in the cylinder and look up and record the literature value of the density of that metal.

 

10. Repeat the measurements for a different letter metal cylinder.

Analysis and Questions (Please use your own paper for the answers of these questions, but number them the same way. Attach this to the data sheet to turn in. Errors in significant figures or missing units will be docked a half point):

 

1. For each piece of glassware, determine the mass of the water, then use the density of water at the measured classroom temperature to find the calculated volume of the water. Clearly, you should enter this information into the data table, 2nd to last line, but show the details of one such calculation.

 

2. Which is more precise, the measured volume or the calculated volume? Why?

 

3. Which is more accurate, the measured volume or the calculated volume? Why? Hint: This should not be the same reason as the one you gave in question 2.

 

4. Calculate the percent error for each piece of glassware, using the more accurate in each pair as the true value. Use the formula for percent error given in class or in the text. Rank the glassware items in order of decreasing accuracy.

 

5. Calculate the density of the metal cylinder using the cylinder dimensions and the formula for the volume of a regular cylinder. Of course, enter the information in the data table, but show the details of your calculation.

 

6. Calculate the density of the metal cylinder using the water displacement method. Again, show the details of your calculation.

 

7. Which method is more precise, using the cylinder dimensions or water displacement? Why?

 

8. Which method is more accurate, using the cylinder dimensions or water displacement? Why? Hint: This should not be the same reason as the one you gave in question 7.

 

9. Calculate the percent error of the metal cylinder density for both the cylinder calculation and water displacement methods, using the density value obtained from a reputable source, such as the textbook.

 

10. Why wonÕt this technique work to measure densities of materials that are less than 1 g/mL?