It represents the number of moles required of that molecule to balance the chemical equation, which means to have the reaction chemically happen and goes to completion.
For example:
CH4 + O2 --> H2O + CO2 that is not balanced
with the coefficients located
CH4 + 2O2 --> 2H2O + CO2 now with the coefficients the number of oxygen and hydrogen on each side are equal
Because you are never adding more than the substances created, nor are you creating any, but should a chemical reaction take place you could see the liquid change form into a gaseous state and that would result a loss of the liquid volume.
So to wrap it all up you can’t have more liquids than what is already there but you could always lose some due to a chemical change, hence the reason it says an open flask, the chemical change would not be collected, mass would be lost
aswer: protactinium-234 is more dang erous than uranium
