Lab 2 Density of Solids and Measurement Challenge

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Great job with your laboratory notebooks last week. Keep up the good work!

Today we are going to practice taking measurements using the techniques we learned in the lesson, plus investigate the density of solids. You will learn to make accurate measurements, estimate to the proper level of certainty, and apply rules for significant figures in calculations.

Experimental Title: Lab 2 Density of Solids and Measurement Challenge

Date of laboratory:  June 10, 2014

Purpose: The purpose of this laboratory is to measure the volume, mass and density of solid substances.

Introduction:  How to measure using estimation.

measurement-ruler

How would you measure the red line in this example? To take a measurement with the ruler above it, first you would count the spaces between the large numbers. There are 10 spaces in the example, so each space is 1/10 of the distance between the black-labeled marks. If the black marks represent centimeters, then each smaller mark is 1 millimeter apart.

The green mark just before the red arrow is 9/10ths of the distance between 7 and 8, which is 7.9 cm (or 79 mm).  Previously you might have reported the answer as 7.9 cm. In chemistry, however, you want to get a more accurate reading of this measurement because the red line actually extends past 7.9 cm. How do you do this when there aren’t any markings? Try to visualize 10 steps in the space between the 7.9 and 8.0. The easiest one to visualize would be 5 steps (5/10) or halfway between.  It is pretty clear the red arrow is less than halfway, so the length of the red arrow is less than 7.95 cm.

Now estimate halfway between 7.9 and 7.95. That would be 7.925, but you can’t see that accurately. The arrow is very close to half of the first half.  So, you could record the length as 7.92 or 7.93 cm, either one would be correct.

In summary, the first two digits (7.9) are measured without any estimation. They make 2 significant figures (also called significant digits). The last digit is an educated estimate, but it does give us more accuracy. Therefore, it is counted as a third significant figure. By estimating, you are getting a little more accuracy than what the markings read.

Don’t worry, this will become easier with practice.

Important equations:

Density can be calculated using the formula:

density= mass (g)/volume(mL or cm3)

Volume of a cube is   V= S3 where S = length of an edge

Volume of a rectangular prism is  V =lwh  where l is the length of the base, w is its width and h is its height

The volume of a triangular prism is V = AH  where A = the area of the triangular base or 1/2bh and H = the height of the prism

Special safety concerns for Lab 2:

  • If anything spills, please clean it up immediately with a paper towel and let your instructor know.
  • If glass breaks, do not pick it up with your bare hands. Notify your instructor immediately.
  • Be sure to wash your hands when you are finished with this lab

Materials:

  • Relational GeoSolid® blocks
  • Plastic blocks
  • Rubber stoppers
  • Sample of metal A
  • Sample of metal B
  • Sample of metal C
  • Sample rock
  • Water
  • Graduated cylinders
  • Table top scales
  • Transfer pipette
  • Rulers
  • Calculator

Procedures:

Note:  Today you can do the parts in any order, so go ahead to another part and come back to finish if you need to do so. No need to wait for materials.

Part 1. Determine the Volume of a CUBE and a Rectangular solid with a ruler

Last week we found the volume of a liquid using a graduated cylinder. This week we are going to find the volume of regularly-shaped objects by measuring and using mathematical formulas. Remember that a cubed centimeter is equal to 1 milliliter.

Procedure 1.

  1. Obtain a Relational GeoSolid® cube.
  2. Measure the length of a side in cm. Verify the shape is a cube by making sure the other sides are the same length.
  3. Record the length in your notebook.
  4. Using a calculator, calculate the volume using the formula V= S3.
  5. Record your answer using the correct number of significant figures.

cube-volume-table6. Repeat using the rectangular prism using the formula V =lwh.

prism-volume-table(Edit) 7. Now check the volumes you obtained by filling the Relational Geosolid® shapes with water.  Pour the water into a graduated cylinder and measure the volume.

Optional 1:  Obtain the solid triangular prism and calculate the volume using the formula V = AH where A = the area of the triangular base or 1/2bh and H = the height of the prism

triangular-prism-volume-tableWeigh the prism to obtain its mass. Measure the sides and calculate the volume. Now calculate the density. According to the text the density of glass is 2.6 g/cm3. Do you think the solid triangular prism is made of glass?

Let’s check to see if liquid volume is really equal to calculated volume.

8. Obtain a solid plastic cube (letter die)
9. Measure three sides to determine if it is a cube. If it is a true cube, then use the formula for volume V= S3 . If not, the use the formula for the volume V =lwh.
10. Record the side lengths and calculated volume.
11. Place 25 mL of water in a graduated cylinder. Carefully drop in the letter cube.
12. Record the final water level. Calculate the volume by subtracting 25 from the final level. How do the two volumes compare?

cube-2volume-table

Part 2. Determine the Density of an Irregularly-shaped object Using Water Displacement

Do you remember the density video from Lab 1? In it the narrator explained how to figure out a volume of an irregularly-shaped solid object by immersing it in water in a graduated cylinder and recording the difference in water level.

Procedure 2.

1. Obtain a small rubber stopper and a graduated cylinder.
2. Weigh and record the mass of the dry stopper. Use the more accurate smaller scale.
3. Use tap water to fill your graduated cylinder to 25 mL.
4. Read and record this volume to the nearest 0.1 mL remembering to read the volume at the bottom of the meniscus.
5. Carefully submerge the rubber stopper in the graduated cylinder.
6. Read and record the new volume. What is the volume of the rubber stopper?
7. What is the density of the rubber stopper?

Leave room to record your observations in your notebook.

Part 3. Identify unknown samples using density

  • Sample of metal A
  • Sample of metal B
  • Sample of metal C
  • Sample rock

Use what you have learned in the part 2 of this lab to calculate the density of metal samples and find out their identity. Be sure to take careful notes of what you do and what your results are.

Repeat procedure 2, replacing the rubber stopper with metal samples.

metals-density-table

Use this table of the density of some common substances to identify your unknowns.

Substance        Density (g/cm3)
Air                          0.0013
Wood (oak)          0.85
Water                     1.00
Ice                          0.93
Aluminum             2.7
Lead                        11.3
Copper                   8.96
Gold                        19.3
Iron                         7.86
Pyrite                       5.0
Galena                     7.5
Zinc                         7.133 – 7.14

(Edit) Optional 2. Measure the length, width and height of a wood block. Calculate the volume using V =lwh. Weigh the wood block. Now calculate its density. Is the wood block more or less dense than oak? Why is it preferable to calculate the volume of the wood block rather than use the water displacement method?

Final optional:  Obtain and weigh 5 dry pennies using the more accurate smaller scale. Now take their volume using the water method and calculate the density. Based on the density you obtained, do you think pennies are pure copper?

Please leave a comment or send an e-mail if you have any questions before our meeting.

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