Mass Balance

Choose a system boundary

Before writing a mass balance, decide which vessel or process section is being studied. Add every mass flow entering and leaving that boundary. If inflow exceeds outflow, mass accumulates. Starting with 200 kg, an inlet of 120 kg/h and an outlet of 100 kg/h leave 240 kg after two hours. The calculator checks whether the inventory would run out during the selected period. It cannot check overflow because vessel capacity is not an input.

Mixing calculation

For two continuous streams, total outlet flow equals their combined inlet flow. The conserved component flow is the sum of each flow multiplied by its component mass fraction. Mixing 100 kg/h at 10 percent with 300 kg/h at 30 percent gives 25 percent. This model is insufficient if there is a reaction, evaporation, an additional outlet or accumulation in the mixer.

Track the component in a separate balance

For mixing, choose the first stream as 160 kg/h at 15 % and the second as 240 kg/h at 35 %. The first carries 160 × 0.15 = 24 kg/h of component. The second carries 240 × 0.35 = 84 kg/h. Total component flow is 24 + 84 = 108 kg/h and total mixture flow is 160 + 240 = 400 kg/h. The final mass fraction is 108 / 400 = 0.27, or 27 %. Because the flows differ, a simple arithmetic average of the inlet percentages is incorrect here.

Confirm that both concentrations refer to the same component and mass basis. Do not enter volume concentration or mole fraction directly as mass percent. A pure solvent stream may have zero component percentage, but its flow still affects the total. An internal recycle that remains inside the system boundary must not be counted again as an external inlet. If flows change during the accumulation period, split the period into intervals with constant flow and use each final mass as the next initial mass. This preliminary calculation is only a mass balance. Mixture temperature, volume change, solubility and overflow safety require separate assessment.

Frequently asked questions

How do you calculate mass balance?

In this example, a tank initially holding 200 kg, with inflow 120 kg/h and outflow 100 kg/h, contains 240 kg after 2 hours. Add net flow multiplied by time to the initial mass, noting that this preliminary balance assumes constant flows and does not check overflow without tank capacity.

Calculate with these values