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A dilute solution of oleic acid contains 0.01 cm³ of oleic acid per cm³ of solution. Using 100 spherical drops each of radius (3/(4*pi))^(1/3) * 10⁻³ cm, a monomolecular film of this solution is spread over an area of 4 cm². The resulting thickness of the oleic acid layer is x * 10⁻¹⁴ m. Find x.

1. 150
2. 200
3. 250
4. 300

Correct answer: 250

Solution

The special radius makes each drop's volume exactly 10⁻⁹ cm³. Multiply by 100 drops, take 1% for the oleic acid, and divide by the film area to get the thickness.

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