Mole have to be used in order to show the molar relationship between the reacting substances. Reacting molecules do not have the same mass, so mole is the basis on which they are measured in order to create a common ground.
Moles quantify the amount of substances that will be consumed or produced in reactions. As every element and compound has a different mass (although sometimes they are the same), it is difficult to deal with masses in chemical reactions.
