In stoichiometry, we look at chemical reactions from the point of view of the quantitative relation between the reactants and the products. In any chemical reaction the number of atoms of elements that make up the compounds that are reacting is the same as the number of atoms of the elements that make up the products.
In analyzing chemical reactions it is the number of atoms of the compounds that is of relevance, not the mass of the chemicals themselves. This follows from the fact that different compounds have different masses and the same mass of two compounds is not likely to contain the same number of molecules or atoms.
This makes it essential to convert quantities from grams to moles. The number of molecules or atoms in a mole is the same for all compounds. Once we have arrived at the number of moles of different compounds that are undergoing a reaction, we can easily determine how much of any compound would be left unreacted, what is the exact reaction taking place, etc.