Some substances that are only very slightly soluble in water will spread over the surface of water to produce a film that is called a monolayer because it is only one molecule thick. A practical use of this phenomenon is to cover ponds to reduce the loss of water by evaporation. Stearic acid forms a monolayer on water. The molecules are arranged upright and in contact with one another, rather like pencils tightly packed and standing upright in a coffee mug. The model below represents an individual stearic acid molecule in the monolayer.

(a) How many square meters of water surface would be covered by a monolayer made from 10.0 g of stearic acid?

(b) If stearic acid has a density of 0.85 g/cm³, estimate the length (in nanometers) of a stearic acid molecule.

(c) A very dilute solution of oleic acid in liquid pentane is prepared in the following way:

A 0.10 mL sample of solution (4) is spread in a monolayer on water. The area covered by the monolayer is 85 cm². Assume that oleic acid molecules are arranged in the same way as described for stearic acid, and that the cross-sectional area of the molecule is 4.6 x 10^-15 cm². The density of oleic acid is 0.895 g/mL. Use these data to obtain an approximate value of Avogadro’s number.

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1.00 mL oleic acid + 9.00 mL pentane → solution (1); 1.00 mL solution (1) + 9.00 mL pentane → solution (2); 1.00 mL solution (2) + 9.00 mL pentane → solution (3); 1.00 mL solution (3) + 9.00 mL pentane → solution (4).