ChemLab #4 - Measuring the Size of a Molecule
Date Submitted:
Lab Group Members:
Introduction: Because atoms and molecules are so small, it is difficult to determine their sizes. Special instruments are normally needed for such measurements. In this experiment, you will determine an approximate measure of the size of a molecule.
Materials:     liquid soap in polypot, chemplate, Lycopodium powder, disposable dropper pipet, pie pan, ruler, water
Experimental Design:
  1. Holding it over the chemplate, collect a drop of soap at the tip of the polypot (without it falling off).
  2. Measure the diameter of the drop (at its widest, assume the drop is a sphere), in millimeters, and record this measurement in the data table. Also record the radius of the drop (r = 1/2 d).
  3. Add water to the pan until it is nearly filled. Using the disposable dropper pipet, spread a thin, even coating of Lycopodium powder over the surface of the water. An uneven layer will increase the experimental error of your results.
  4. Add a single drop of soap to the center of the pan. Note what happens to the powder.
  5. Use the boundary of the powder to measure the diameter of the circular soap layer on the surface of the water. If the soap layer is not a perfect circle, measure the widest area. Record the diameter and radius of the soap layer on the data table.
 
Data Table
Soap
Measured
Diameter (mm)
Calculated
Radius (mm)
Calculated
Height (mm)
Calculated
Volume (mm3)
 
Drop (sphere)
  		       
 
Layer (cylinder)
  		       
 
 

Calculations: (Show all calculations, including equations, on the back of the report form.)
  1. The drop will be close to the shape of a sphere as it leaves the dropper. Calculate the volume of the drop. (The volume of a sphere of radius r is 4/3 π r3). Record this volume in the data table.
     
  2. Although the soap layer appears to be a flat circle, it actually has some depth, making the soap layer a cylinder. Assume the height of this cylinder is the thickness of a single soap molecule. The value of the height (or approximate thickness of the soap molecule) can be calculated from the formula for the volume of a cylinder: V = π r2h. This equation can be rearranged to solve for the height h = V/π r2. Note that the volume of the soap is the same whether it is in the form of a sphere or a cylinder. Substitute the value for the volume of the soap drop and the value for radius of the soap layer into the rearranged equation, and solve for the height of the soap layer. Record the height in the data table.
 
Conclusion:     The approximate diameter of a soap molecule is __________ mm.