'VISIBLE" MONOMOLECULAR FILMS Estimation of Molecular Size and Avogadro's Number Submitted l>y: L. Carroll Kmg, Northwestern T^nversit,y, 'Kvanston, Illinois Checked by: James R. Mills, Southfield High School, Sont,hfidd, Michigan
PREPARATION
Set u p a series of four test tubes each containing 9.0 ml of pcnt,ane. Add 1.0 ml of oleic acid to t,he first, t,ransfer 1.0 ml of this solution to tube 2, and so on. Tube 4 will contain 0.001 ml of o l ~ i cacid per 10.0 nll of solution. A plastic cake dish. 15 X 20 X 9 em, is filled with wat,er until the water surface is above the edge of the dish; add one drop of concentrated hydrochloric acid to the water Clean the water surface wit,h a lucite barrier, 17 X 2 X 1 em, cover with "piston oil," and dust with lycopodium powder. DEMONSTRATION
Place 0.10 ml of the solution in tube 4 (1 X 10"' cm3 of oleic acid) on the surface of the measuring t,ray. At'ter evaporation of the pentane, the oloic acid film is
easily visible since it will push back the piston oil and lycopodium powder and form an approximately circular area on the water surface. REMARKS
The area of the film may himeasured by outlining the film on a glass plate, transferring the outline to a piece of graph paper graduated in millimeters, and counting the squares. From the known volun~eof oleic acid in the film and from its area. the thickness of the monomolecular film can he calculated. Calculation of Avogadro's number from this data was discussed by King, L. C., and Xeilsen, E. K., J. CHEM.Eurc., 35, 198 (1958). "Piston oil" may be prepared by heating 'Hyvac" oil for eight hours a t 300° or until it forms a film. [See also Kuchner, A. I.., J. CHEM.Eurc., 19, 27 (1942)l.
m
l of Chewteal Education
Oitober 1981
FIRE UNDER WATER Submitted by: It. B. Esciic, North Texas State University, Denton, Texas. Checked by: Berta ^\fay Pope, Arlington High School, A.rlingt,on, Texas.
PREPARATION
Fill :I conical vessel or a 100-ml graduated cylinder (size 3) almost full of water. Add sufficient potassium chlorate without stirring to form a layer of the solid about F) mm thick a t the bottom of the vessel. Cut. a piece of white phosphorus the size of half a pea into 4 or 5 smaller pieces. Drop into t,hc graduate so that the phosphorus remains on t,op of the KC103 layer. DEMONSTRATION
1 ' 1 a~ long ~ ~ thistle tube so that. its outlet is near the phosphorus pieces and slowly pour separate small (0.5 nil) port.ionsof concentrated sulfuric acid into it. Allow t,hc reaction to diminish after each addit,ion. Soon hubbies of gas will form that will cause sparks as they con-
tact the phosphorus. The "tire" or sparks readily seen in a darkened room are produced nndrr watrr. REMARKS
The gas prohahly is a mixture of chloinic dioxide and oxygen 4KC10, 2H3SO. 2K,S04 + 4 C109 0. + 2H20
+
-
+
Do not add any fresh potassium chloratc to the acidifiod mixture. When the demonstration is concluded, decmt most of the fluid without exposing the phosphorus to air. Add fresh water to dissolve the potassium chlorate residue. Continue decanting and washing until only phosphorus remains for disposal by burning.
