Name______Date ______Period ______

Density Determination & Percent Error Lab

Purpose

Determine the density of a metal by experimental methods (displacement of water) and find the percent error in measurements made in the lab versus accepted density values. After completing the data collection, you will plot mass and volume data sets on an Excel spreadsheet to determine the relationship between these measurements. The trend that is revealed and its application will be analyzed and used to identify the substance.

Background

The mass of a solid can be easily measured using an electronic balance. The volume of an irregularly shaped solid is extremely difficult to determine directly. Instead, its volume is measured by an indirect method called water displacement. The initial volume of a given amount of water is measured using a graduated cylinder. The solid is then carefully added to the water in the graduated cylinder and the new (final) volume is recorded. The volume occupied by the solid must be the same as the volume of water that has been displaced and is therefore equal to the difference between the final and initial volumes.

Materials

Metal Shot, 25-30 gramsBalance, electronic (0.01 g precision)

Graduated cylinder, 100-mLWeighing dishes or small containers

Paper towelsWater

Pen for labeling

Procedure

  1. Obtain a sample of metal shot, 25-30 grams.
  1. Label a set of weighing dishes or small containers 1-5.
  1. Tare (“zero”) weighing dish #1 on the electronic balance. And add about one-fifth of the metal shot to the dish. Measure the mass of sample of sample #1 (it should be between 4 and 6 grams). Record the mass of sample #1 in the Data Table.
  1. Repeat step 3 to divide the metal shot among the other four weighing dishes. Vary sample sizes so they are not all the same mass. Thus, if the first sample is 6 grams, make the next sample about 4 grams. Do not mix up the samples.
  1. Obtain a 100-mL graduated cylinder and add approximately 30 mL of water to the graduated cylinder.
  1. Measure the initial volume of water in the cylinder to the nearest 0.1 mL and record the value for sample #1 as “Volume (initial), mL” in the Data Table. Note: Use the units mL for the volume measurements. Recall, 1 mL = 1 cm3.
  1. Carefully add sample #1 to the water in the graduated cylinder. The best way to do this is to tip the cylinder at a slight angle and gently slide the metal pieces into the water so that the water does not splash or splatter (and the glass cylinder does not break). Record the final volume (volume of water plus the sample) in the Data Table.
  1. Subtract the initial volume from the final volume to calculate the volume of sample #1. Record this value as “Volume (sample), mL” in the Data Table.
  1. Repeat steps 6-8 for each of the four remaining samples. Do NOT remove prior samples from the cylinder between measurements. Before adding a new sample to the cylinder, measure the new “initial” volume in the graduated cylinder. This may not always be precisely the same as the previous final volume reading. Record initial and final volume measurements and the volume of each subsequent sample in the Data Table.

Data Table

Volume Volume Volume

SampleMass, g(initial), mL(final), mL(sample), mL

1

______

2

______

3

______

4

______

5

______

Graphing the Data

Plot the mass and volume data for samples 1-5 on an Excel spreadsheet. Plot the mass on the x-axis and the volume on the y-axis. Each sample is represented by one point. Label each axis-don’t forget the units- and make sure the scale is clearly marked. Do NOT play “connect-the-dots” with the data points. Use a trendline to determine the “best-fit” line for your data points.

Post-Lab Questions

  1. Does it make sense that any trend or pattern in the data should include (0,0) as a point? Explain your reasoning.
  1. What kind of trend or pattern is obvious in the data? Is there a consistent relationship between the volume and mass measurements? Explain.
  1. Calculate the slope of the “best-fit” line. Show all of your work! What are the units of the slope? What physical property is represented by the slope of this line?
  1. Compare the value of the slope you determined with one other student group. Are the values of the slope similar?
  1. Use the following equation to calculate the percent error in your measurement of the slope and the physical property it represents. The percent error measures the accuracy of your results. Comment on the accuracy of this procedure and identify possible sources of error.

measured value – accepted value

percent error = ______x 100%

accepted value

  1. Density can be calculated directly by dividing the mass of an object by its volume. Using the mass and volume measurements recorded in the Data Table, calculate the density for each sample, the average density, and the difference between each density value and the average value.

Density of Sample #1 ______Density of Sample #2 ______

Density of Sample #3 ______Density of Sample #4 ______

Density of Sample #5 ______

Average Density of Samples______

  1. If a student used the“Volume (final) mL” of the graduated cylinder after the metal had been added instead of the “Volume (sample) mL,” how would the density of the metal increase or decrease? Explain your answer.