What is Mass flow rate?
The mass of gas flowing per second (kg/s).
In steady flow, mass is neither created nor lost. Volume flow changes with temperature and pressure, but mass flow does not, so it is the natural basis for pipes and reactors.
Heat a stream of air and it expands, so its volume flow rises — yet the mass passing each second stays the same.
Enter kg/s, kg/h or g/s. A mixer's outlet flow is the sum of its inlet flows; a PFR uses the flow to compute residence time.
\dot{m} = \rho\, u\, A
with density \rho, velocity u and cross-section area A.