What is the first step in most stoichiometry problems?

Explanation:

Stoichiometry is the cuantitative study of the chemical reactions.

It is like algebra applied to chemical equations.

The cuantitative relations between the amount of reactants and products is determined by the law of conservation of mass: the number of each kind of atoms in the reactants must equal the number of the same kind of atoms in the products.

Once that relation has been established, as mole ratios, then it can be determined the amount of reactant neeed to obtain a certain amount of product, or vice versa, determine the amount of product that can be obtained from a given amount of reactants.

That is why, after you know the reactants and products in a chemical equation you must balance to assure that the relative amounts are properly established.