In stoichiometry, why is it neccessary to convert quantities to moles rather than compare grams?

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.