How Big Is a Molecule?. Purpose : Demonstrate the use of scientific notation, significant figures, and units by estimating the size of a soap molecule
Purpose: Demonstrate the use of scientific notation, significant figures, and units by estimating the size of a soap molecule
When working and thinking in the size range of molecules, numbers that are both very tiny AND gigantic in magnitude are encountered. There is a convenient way to express these numbers.
This demonstration provides a fun way to work with uncommonly big and small numbers while at the same time calibrating your thinking about the dimensions and size scale of the world at the molecular level.
Here’s a model of a typical soap molecule…
and…How many in a tiny drop?
Side view of soap film
Height: 1 molecule “thick”The Data
Basically, the soap film forms a cylinder shape that is 1 molecule thick.
If we can calculate the thickness of the cylinder, then we will know
roughly how long the soap molecule is.
Convert inches to cm
(16.9 inches)( 2.54 cm/1 inch) = 42.926 cm
Is this ok ??? Because my calculator says so?
We need to ROUND to the correct sig figs
Answer: 42.9 cm (the conversion factor does not
limit the sig figs to 3 in this case because 2.54 is
EXACT by definition)
Finally, the height of the cylinder, i.e., the length of the molecule
RECALL : h = V/r2
h =(0.00100 cm3)/ (42.9 cm)2
Unit analysis: volume/area = length
h = 0.000000173 cm
Answer: h = 1.73 x 10-7 cm