Laboratory Activities

Laboratory Assessment Activity B.5

The Problem: Devise and carry out an experiment to determine the density of the plastic in a Beral-type pipet. Describe the method you developed to solve this problem.

Assessment Objectives

This problem tests a student's ability to determine the density of an irregularly shaped item. It also allows for a variety of approaches, giving students an opportunity to demonstrate their creativity in problem solving.

One Likely Approach

1. The student weighs an empty Beral-type pipet.
2. The student fills the pipet completely with water, making sure that there are no air spaces in the bulb or in the stem. The student finds the mass of the pipet full of water and subtracts the mass of the empty pipet. The mass of the water in grams is approximately equal to the volume of water in mL at room temperature.
3. The student finds the volume of the pipet itself by displacement of water. The student then subtracts the internal volume from the total pipet volume to find the volume of the pipet itself.
4. The student calculates the density of the pipet from the mass and volume data.

Scoring Suggestions (Based on 5 Points)

1. Determination of the mass of the pipet. 1 pt
2. Determination of the volume of the pipet 3 pt
   1. The student determines only the internal volume of the pipet 1 pt
   2. The student uses displacement of water to find the volume of the pipet but fails to subtract the internal volume. 2 pt
   3. The student uses the likely approach procedure to find volume. 3 pt
3. Calculation of density for the pipet. 1 pt

Extra credit could be awarded if the student

1. develops an innovative method of determining the volume of the pipet.
2. repeats measurements rather than relying on a single value.
3. discusses assumptions and sources of error in measurements

Materials & Equipment

Notes

1. Students can solve the problem without destroying the Beral-type pipets. However, students may decide to cut up the pipet in order to measure the volume of the plastic. It is also possible to collapse the pipet by heating and then to measure the volume of the plastic by water displacement.
2. Any size Beral-type pipet will be suitable, but match the size of the pipets used to the size of the cylinders provided. 25-mL cylinders may be needed if large size pipets are used.
3. Most commercially available Beral-type pipets are made from polyethylene with a likely density range 0.93 - 0.97 g/cm3.

Special Safety Considerations

1. None

Your Usage Log & Notes