To calculate the area per molecule occupied by a monolayer of n-octadecane molecules adsorbed at the air-water interface, we need to use the Langmuir adsorption isotherm equation: = k * A * / 1 - where is the surface pressure 20 mN/m , k is the Boltzmann constant 1.38 10^-23 J/K , A is the molecular area in the liquid phase 0.219 nm , and is the surface coverage fraction of the surface covered by the adsorbed molecules .First, we need to find the value of . Rearrange the Langmuir equation to solve for : = / k * A + Now, we need to convert the surface pressure from mN/m to J/m: = 20 mN/m * 1 N / 1000 mN * 1 J / 1 N*m = 0.02 J/mNow, we can plug in the values into the equation: = 0.02 J/m / 1.38 10^-23 J/K * 0.219 10^-18 m + 0.02 J/m 0.02 J/m / 3.02 10^-22 J/m + 0.02 J/m 0.02 J/m / 0.0200000302 J/m 0.99999849Now that we have the surface coverage, we can calculate the area per molecule occupied by a monolayer of n-octadecane molecules adsorbed at the air-water interface:A_adsorbed = A_liquid / 1 - A_adsorbed = 0.219 nm / 1 - 0.99999849 A_adsorbed 0.219 nm / 1.51 10^-6 A_adsorbed 145.03 nmSo, the area per molecule occupied by a monolayer of n-octadecane molecules adsorbed at the air-water interface is approximately 145.03 nm.