Question

10 drops of an oleic acid solution of concentration 1/400 ${cm}^3$ per ${cm}^3$ of alcohol are dropped on a water surface. The circular film thus produced has radius 10 cm. Find the molecular size of oleic acid , if the radius of each drop is 1.4 mm.

Answer 1

Number of drop=10

Concentration=$\frac{1}{400}{cm}^3$

Volume of oil drop=

$=10\times \frac{4}{8}\pi \times \frac{1.4\times 1.4\times 1.4}{1000\times 1000\times 1000}\times \frac{1}{400}$

$=\frac{10\times 4\pi \times 1.4\times 1.4\times 1.4}{8\times 1000\times 1000\times 1000\times 400}$

$=\frac{10\times 4\pi \times 2.744\times {10}^{-11}}{12}$

Area of circular film of radius r

$=10cm=0.1m$

$Area=0.1\times 0.1\pi$

$=0.01\pi m^2$

Thickness of molecular size

$=\frac{volume}{area}$

$=\frac{10\times 4\pi \times 2.744\times {10}^{-11}}{12\times 0.01\pi }$

$=9.1\times {10}^{-9}m$