Stearic Acid Monolayer

Question

1. A certain acid (molecule length = 11 Å) has a density of 0.92 g/cm^3. If a monolayer of the acid is formed on the surface of water on a 5.25 cm diameter watch glass, What is the mass of acid in the monolayer?

Base area of monolayer (in cm) (show work below)

Volume of monolayer (in cm^3) (show work below)

Mass of acid in monolayer (in g) (show work below)

2. The above acid has a molar mass of approximately 170 g/mol. How many molecules are there in the monolayer described above?

Answer 1

Stearic acids have hydrophilic heads and hydrophobic tails, most likely they all stick their hydrophilic heads to the water and set the tails vertical, so the monolayer thickness is about the length of the acid chain.

11A = 10^-8cm

V = 5.25 cm²*11 x 10^-8cm = 5.8x10^-7 cm³

m = V * d = 5.8x10^-7 x 0.92 g/cm³ = 5.3x10^-7g