Experiment: Mass, Volume, Density
and Significant Figures

Purpose: To learn how to use a centigram balance, metric ruler and graduated cylinder to the correct number of significant figures and to determine the densities of several materials to the correct number of significant figures.

Background: An old riddle asks “Which is heavier: a pound of feathers or a pound of lead?”  The question is nonsensical, of course, since a pound of feathers and a pound of lead both weigh the same – one pound.  Nevertheless, there is a clearly something different about a small lead brick and a large bag of feathers, even though they weigh about the same.  The key to answering the riddle is understanding the relationship that exists between a substance’s mass and the volume it occupies.  This relationship is expressed by the physical property called DENSITY.

Density can be calculated as equal to the mass of substance (in grams) divided by the volume of the substance (in mL).  In this experiment, you will measure the mass and volume of several samples.  Some will have a regular shape, others will not.  You will use several different tools and record the values obtained to the correct number of significant figures.  Remember, the degree of precision of the measuring instrument dictates the place to which you record your answer.  You will then use your data to calculate the density of each sample - again, using your knowledge of significant figures as they apply to measured numbers in calculations.  Finally, you will be given the accepted values for the densities of your samples and you can then calculate the % error of your work.

Materials:

• centigram balance

• graduated cylinder

• metric ruler

• substances:

  • block of wood

  • paper clips

  • glass marble

  • tap water

Procedure:

REMINDER: Use blue or black PEN on data tables.

1. From the cabinet, each lab group will get a centigram balance.  First, be sure it is properly balanced - the right hand side should line up with the appropriate line.

2. Determine the calibration of the centigram balance and record in your data table #1.  Remember, a calibration includes units! Then, complete the 3rd column.

3. Get a graduated cylinder from the front table. Determine the calibration of the graduated cylinder and record in your data table #1.  Then, complete the 3rd column.

4. Get a metric ruler.  Determine the calibration of the metric ruler and record in your data table #1.  Then, complete the 3rd column.

5. Get one wood block, 10 large paper clips and a piece limestone.

• WOOD BLOCK

  • Using the correct number of significant figures, record the mass of the wood block on data table #2.

  • Using the correct number of significant figures, record the length, width and height measurements of the wood block.  Record on data table #2.

• PAPER CLIPS

  • Using the correct number of significant figures, record the mass of all 10 large paper clips on data table #2.

  • You will used the method of water displacement to determine the volume of the 10 paper clips.  Using the correct number of significant figures, record the starting and ending volume on data table #2.  Calculate the net volume.

• GLASS MARBLE

  • Using the correct number of significant figures, record the mass of several marbles on data table #2.

  • You will used the method of water displacement to determine the volume of the glass marbles  Using the correct number of significant figures, record the starting and ending volume on data table #2.  Calculate the net volume.

• TAP WATER

  • Using the correct number of significant figures, record the mass of your DRY graduated cylinder on data table #2.

  • Fill graduated cylinder with about 5 mL of water but record EXACTLY (to the correct number of significant figures) your water volume on data table #2.

  • Place the graduated cylinder with the water onto the centigram balance.  Record the total mass.  Then, calculate the net mass (to the correct number of significant figures).

6. Dry off any wet samples and return all materials to the front storage cart.

7. Calculate the densities of all four samples.  Pay attention to the correct number of significant figures for each calculation.  Use the rules for calculation that you were taught.

Data Tables:

• Print off the DATA TABLES by clicking on this link.  You will use these two data tables in the lab to record your data.
• Remember that all calibrations include units.
• Remember that when you record a measurement, you must include units.
• Remember that when you set up a calculation on your paper, each numerical value must include units.
• In the lab, be sure to use blue or black PEN (NO PENCIL) when you complete this data table. 
• NEVER WHITE-OUT data. 
• Enter data carefully so that you make no mistakes. 
• Print neatly!

Analysis:

Word process the answers to these questions.   Turn in your data tables (from previous sections) along with the answers to the questions below when the experiment write-up is due. Answers to these questions must be word processed!

1. Think carefully about your answer to this question: For YOU, what was the most challenging part of this experiment?  Detail your response.

2. Why do we use water displacement for the paper clips and the glass marbles to measure volume?

3. You will calculate the PERCENT ERROR in the density calculations for the 4 samples.  Your teacher will provide you with the accepted value for the density of each sample.  What formula is used to calculate percent error? 

4. Using your formula above for #3, create and insert a new table (for YOUR work) and calculate % ERROR for each sample.  You may WRITE in your work onto the table you created (instead of word-processing) showing ALL WORK as you do your calculations.  Discuss the error you got for each sample.  Review actual values for each sample from the table above.  Was it a lot, a little?  Explain your results.  If you had a lot of error, WHY do you think your results were so bad?  What would you do differently?

5. There are two different meanings of the word PRECISION as we have discussed in class and in this experiment.  What are the two different meanings of the word PRECISION?  Google this word.  Detail your response.

Sources:

• http://cnc.hkcampus.net/~cnc-is/labequipment.html
• Prentice-Hall Chemistry 2002