Which quantity is used to express the rate of mass transfer per unit area in a flowing system?

• A. Mass flux
• B. Mass concentration
• C. Diffusion coefficient
• D. Gas constant

Answer: (A)

Mass flux is the rate at which mass crosses a surface per unit area per unit time. In a flowing system, it captures how quickly material is transferred across a boundary due to the motion of the fluid (convection) and can include any diffusive contributions in a complete transport description. Its standard units are kilograms per square meter per second, reflecting the crossing of area over time. This stands in contrast to mass concentration, which is mass per volume and does not specify transfer rate; the diffusion coefficient describes how readily species spread but is a property that relates flux to a concentration gradient rather than giving the transfer rate itself; and the gas constant is just a fundamental constant with no direct transport meaning. In a simple convective flow, the mass flux across a surface can be thought of as ρ times the normal velocity, illustrating why flux is the quantity that directly expresses mass transfer per unit area.