# Molecular diffusion question; fick's first law

• Chemistry
• aquabum619
In summary, the flux of oxygen across a square centimeter membrane is -30 [mol]/[m^2 sec], taking into account the molecular diffusion constant and the thickness of the membrane. This is calculated using Fick's first law of diffusion and the given concentrations on either side of the membrane.
aquabum619

## Homework Statement

Calculate the flux of oxygen across a square centimeter membrane [2$$\mu$$, or 2x10 $$^{-6}$$m thick if the concentration on the right hand side of the membrane is 4mL/L and on the left side is 2mL/L.

## Homework Equations

fick's first law of diffusion
Flux = dC/dx = D[C2-C1] where D is the molecular diffusion constant
molecular diffusion constant for oxygen = 3x10^-5

## The Attempt at a Solution

Using the first eq, I took the difference in the concentrations, and divided it by the thickness of the membrane. [4mL/L-2mL/L]/ 2$$\mu$$, which resulted in a flux of 1x10$$^{6}$$... however, the equation did not take into account the molecular diffusion constant of oxygen, which was provided in the problem. So then I tried the second equation. 3x10$$^{-5}$$x[4mL/L - 2mL/L]= 6x10$$^{-5}$$. however, the second equation does not take into account the thickness of the membrane (dx). I am totally lost and have no idea what to do. please help thanks!

Last edited:
You're almost there, but your flux equation looks a little off. Make sure to use the right equation and work through the problem again using the correct units, and things should work out fine.

after doing a little research, I come to realize that the diffusion equation (Fick's first law) is actually

Flux = -D[($$\Delta$$C)/($$\Delta$$x]
where D is the diffusion coefficient;
where C is the concentration
where $$\Delta$$x is the thickness of the membrane

therefore, -3x10^-5 [(2mL/L)/(2x10^-6m)]= -30 [mol]/[m^2 sec]

Last edited:

## 1. What is molecular diffusion?

Molecular diffusion is the process by which molecules move from an area of higher concentration to an area of lower concentration. It occurs due to the random motion of molecules and is an important mechanism for the transport of substances in various biological and chemical processes.

## 2. What is Fick's first law?

Fick's first law, also known as the law of diffusion, is a mathematical equation that describes the rate of molecular diffusion. It states that the rate of diffusion is directly proportional to the concentration gradient and the diffusion coefficient, and inversely proportional to the distance traveled.

## 3. What factors affect molecular diffusion?

The rate of molecular diffusion can be affected by several factors, including temperature, molecular size, and the nature of the medium. Higher temperatures generally result in faster diffusion, while larger molecules diffuse more slowly. The type of medium, such as air or water, can also influence the rate of diffusion.

## 4. How is Fick's first law applied in real-world scenarios?

Fick's first law is commonly used in various fields of science, such as biochemistry, pharmacology, and environmental science. It can be applied to understand the diffusion of gases through membranes, the movement of molecules in a solution, and the transport of substances in living organisms.

## 5. What are some limitations of Fick's first law?

While Fick's first law is a useful tool for understanding molecular diffusion, it has some limitations. It assumes that the medium is uniform and the concentration gradient is constant, which may not always be the case in real-world scenarios. Additionally, it does not account for factors such as convection and chemical reactions, which can also affect the rate of diffusion.

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