In an isothermal, steady-state reactor with a single reacting species, which statement represents the material balance?

• A. Input − Output + Generation = Accumulation
• B. Input + Output − Generation = Accumulation
• C. Accumulation = Input − Output
• D. Accumulation = Output − Input

Answer: (B)

In a reactor at steady state, the amount of the reacting species inside does not change, so the accumulation term is zero. The material balance links what flows in, what flows out, and what the chemistry does to the species inside. If Generation is taken as the rate of production by the reaction, the standard form is Input − Output + Generation = Accumulation, and with accumulation zero this enforces the steady-state balance.

The statement you’ve got uses a convention in which Generation is defined with the opposite sign, so it writes the balance as Input + Output − Generation = Accumulation. It’s the same underlying conservation of mass, just a different sign convention for the generation term. The key idea is that accumulation is zero at steady state, and the equation you use must be consistent with how Generation is defined in the problem.