Explain how the concept of a limiting reagent determines the theoretical yield of a product in a chemical reaction, and outline how a chemist would identify which reagent is limiting when given the masses of two reactants and a balanced equation.

The limiting reagent is the reactant that is completely consumed first, i.e. the one present in the smaller stoichiometric amount, so the reaction stops when it is used up.

The theoretical yield of the product is calculated from the amount of the limiting reagent alone. Using the balanced equation, the moles of product that can be formed are found from the stoichiometric mole ratio between the limiting reagent and the product.

To find which reagent is limiting, a chemist converts the given masses to moles by dividing by the molar mass of each reactant. Then each mole value is divided by its stoichiometric coefficient in the balanced equation. The reactant that gives the smaller quotient is the limiting reagent.