ESTIMATING SMALL SIZES
NAME ____________________________ PARTNER__________________________ DATE______
PURPOSE
(1) To determine the order of magnitude of the size of an oleic acid molecule.
(2) To estimate the number of molecules in one mole of a molecular substance.
MATERIALS
Large tray; beads; dropper; 10 ml, 100 ml graduated cylinders; meter stick; oleic acid in alcohol solution, 1 part in 500 Oleic Acid; lycopodium powder or fine talcum powder.
INTRODUCTION
Molecules are much too small to be observed directly. However, because some molecules will form layers, that are only one molecule thick it is possible to determine the order of magnitude of the size of such molecules with ordinary laboratory equipment. Oleic acid is an oily substance that is insoluble in water and has very low surface tension.
A mole of a molecular substance is the quantity of the substance that is equal to its molecular weight expressed in grams. The number of molecules in a mole of a molecular substance is an important physical constant known as the Avogadro number. Its value is 6.0219 x 1023 molecules per mole. In this experiment we shall also attempt to determine the Avogadro number.
PROCEDURE:
I. Simulation
1. Pour the beads onto the tray and into a confined area until the area (circle or rectangle) is covered with a single thickness layer. Be sure the beads are tightly packed. Measure and calculate this area.
2. Carefully pour the beads from the tray into a graduated cylinder and measure the total volume.
3. Carefully put the beads back into the original container.
II. Real Molecules
4. Thoroughly clean the tray. Pour water into a tray to depth of about half centimeter and dust the water surface very lightly with lycopodium powder or sulfur powder.
5. Put one drop of oleic acid solution on the water surface. The oleic acid pushes the powder outward so that the area of the oleic acid layer is visible. Make measurements of layer and calculate its area.
6. Experimentally determine the number of drops of oleic acid solution per cubic centimeter.
7. Repeat steps four through six two more times.
! DATA/RESULTS
Volume of the Beads Color of Beads Diameter of Circle
*______ Area Of Circle
*______ Height of Bead
*______ Volume of a Bead
*______ Number of Beads Used
spheres
cubes
DROPS in a cm3 Volume of a Drop
*______ Volume/500 Area on Surface
*______ Height of Layer
*______ Volume of a Molecule
*______ Molecules per Mole
*______
CALCULATIONS
1. Use the data to find the diameter (height) of a single bead, from this find the Volume of a single bead;
a) assuming the beads are spheres,
b) assuming the beads are cubes.
2. Use the two volumes to calculate how many beads you put in the graduated cylinder. Do not count beads.
3. Calculate the volume of one drop of solution. Using the dilution ratio of the solution, 1:500, compute the volume of oleic acid in the drop of solution used. This is the volume of the surface layer. Using this volume and the area of the layer, calculate its thickness.
4. Use the thickness of the oleic acid layer on water as the height. Calculate the volume of a single oleic acid molecule assuming the molecules are cubes or spheres (your choice or do both and compare).
5. The mass density of oleic acid is 0.895 g/cm3 and one mole has a mass of 282 g. Calculate the volume occupied by one mole of oleic acid. (Hint: Mass / Density = Volume) From this value and the volume of a single oleic acid molecule, calculate the number of molecules in a mole of oleic acid. Compare your results with the accepted value for the Avogadro number.
6. Go back and check to be sure you did not record too many significant figures.
QUESTIONS/CONCLUSIONS
1. Which assumption of shape (spheres or cubes) do you think gave a more accurate determination of the number of beards used? Explain?
2. How can you be sure the oleic acid, and not the alcohol, is pushing the powder aside? Did you test it?
3. Does this experiment give consistent results? (How did you check this?)
4. What measurable properties of oleic acid make it desirable for this experiment? ( as opposed to just cooking oil, dish soap, gasoline, just alcohol etc.)
5. To obtain a height you could measure with a ruler, how many molecules
or layers should be stacked on top of each other?
And:
Physics
Simulation: Rutherford Model of the Atom
Objective: To Simulate Rutherford's gold-foil scattering experiment, and determine the shape of an "atom" hidden by two boards.
Materials: marbles (alpha particles), grooved ruler, scattering box (atom), large sheet of paper
Procedures
1. Place your atom on the large sheet of paper.
2. Allow the alpha particle to roll down the groove of the ruler such that it enters the atom cleanly and exists the atom cleanly.
3. Draw a line to mark the incident alpha particle and the deflected alpha particle. Number these lines I1 and D1.
4. Repeat approximately 50 times from all directions, labeling each subsequent incident and deflected alpha particle I2, D2, etc.
5. Connect the incident and deflected paths for all I and D values by extended the line back into the atom until they meet.
Analysis
1. Based on the connecting points, propose a "shape" for your atom actual size in your lab book.
2. What are the similarities and differences between this lab and Rutherford's gold-foil experiment?