Question

Which law is used to describe steady-state diffusion in gases?

• A. Henry’s law
• B. Raoult’s law
• C. Fick’s first law
• D. Dalton’s law

Hint:

For non-steady state diffusion (where concentration changes with time), we use Fick's Second Law: \( \frac{\partial c}{\partial t} = D \frac{\partial^2 c}{\partial x^2} \).
Steady state \( \implies \frac{dc}{dt} = 0 \).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Diffusion is the net movement of molecules from a region of higher concentration to a region of lower concentration due to random molecular motion.
Steady-state diffusion occurs when the concentration gradient does not change with time.
Step 2: Key Formula or Approach:
Fick's first law relates the diffusive flux to the concentration under the assumption of steady state.
The mathematical expression is:
\[ J = -D \frac{dc}{dx} \]
Where:
\( J \) = Diffusion flux (amount of substance per unit area per unit time).
\( D \) = Diffusion coefficient or diffusivity.
\( \frac{dc}{dx} \) = Concentration gradient.
Step 3: Detailed Explanation:
- Henry's Law describes the solubility of a gas in a liquid being proportional to the partial pressure of that gas above the liquid.
- Raoult's Law relates the vapor pressure of a solvent in a solution to its mole fraction in the liquid phase.
- Dalton's Law states that the total pressure of a mixture of non-reacting gases is equal to the sum of the partial pressures of the individual gases.
- Fick's First Law is the fundamental law governing the transport of mass by molecular diffusion under steady-state conditions in gases, liquids, and solids.
Step 4: Final Answer:
Fick's first law is the correct law used to describe steady-state diffusion.