How does the mass before a reaction compare to the mass after a reaction according to the law of conservation of mass?
They're the same!
The law of conservation of mass states that, when we're in an isolated system, total mass is constant before and after a chemical reaction or physical change.
In other words, mass is conserved—hence its name being the "law of conservation of mass." :)
Let's look at an example of a burning log:
The total mass before the combustion reaction, which is a chemical reaction, is mostly in the form of wood.
The mass of the log after the reaction, although it might seem as if it just basically disappeared, is conserved in the form of carbon dioxide and water vapor produced as the log burned.