Question

An oil drop of average diameter 0.7mm spreads out into a circular patch of diameter 75cm on the surface of water in trough.

1. Calculate the average thickness of a molecule of oil
2. State two assumptions made in (1) above when calculating the thickness of the oil molecule 

Answer 1

1. Volume of the oil drop 
   = ⁴/₃π³  =   ⁴/₃ x 3.14 x 0.035³ cm³ 
   let the thickness be    =  x
   volume of the patch   =  π R²x
   = 3.14 x (37.5)² cm²  × x 
   Volume of the patch  = volume of the oil drop.
   3.14x (37.5)² cm²  × x  =  ⁴/₃ x  3.14 x (0.035)³ cm³
   x =  ⁴/₃ x (0.035)³cm³  x     1    
                                         (37.5)² 
   = 4.066 x 10⁻⁸cm
   = 4.066 x 120⁻¹⁰ m 
   = 4.1 x 10⁻¹⁰ m.
2.  
   1. the oil does not evaporate 
   2. the oil spreads to a one molecule thick layer.