Equimolar counterdiffusion is defined by Na = -Nb. Which of the following expresses this relation?

Equimolar counterdiffusion is defined by Na = -Nb. Which of the following expresses this relation?

• A. NA = NB
• B. NA = 0
• C. NA = Na
• D. NA = -NB

Answer: (D)

Equimolar counterdiffusion means the two species diffuse with equal magnitudes but in opposite directions, so the molar fluxes cancel each other. This gives the relation N_A = -N_B, meaning the flux of A is the same size as the flux of B but in the opposite direction. A useful consequence is that the total molar flux N = N_A + N_B is zero, reflecting no net bulk movement of the mixture. This is what makes the equimolar case distinct from diffusion in the same direction (which would require N_A = N_B) or from a situation where one species isn’t diffusing (which would not satisfy the equal-opposite condition).

Equimolar counterdiffusion means the two species diffuse with equal magnitudes but in opposite directions, so the molar fluxes cancel each other. This gives the relation N_A = -N_B, meaning the flux of A is the same size as the flux of B but in the opposite direction. A useful consequence is that the total molar flux N = N_A + N_B is zero, reflecting no net bulk movement of the mixture. This is what makes the equimolar case distinct from diffusion in the same direction (which would require N_A = N_B) or from a situation where one species isn’t diffusing (which would not satisfy the equal-opposite condition).