What goes in must come out
Atoms are not created or destroyed in a process — they're just rearranged and moved around. So if you add up all the mass flowing into a piece of equipment, it must equal all the mass flowing out.
Engineers call this a mass balance. It sounds obvious, but it's the workhorse check for the whole field: if your in and out don't match, you've made a mistake somewhere.
Mass in = mass out. Always.