To determine the limiting reactant when the masses of reactants are the same, we need to compare the moles of each reactant based on their stoichiometric coefficients in the balanced chemical equation. The reactant that produces fewer moles of products is the limiting reactant.

For example, consider the reaction between hydrogen (H2) and oxygen (O2) to form water (H2O):

2H2 + O2 -> 2H2O

If we have 1 mole of H2 and 1 mole of O2, we can calculate the moles of water produced by each reactant using their stoichiometric coefficients:

Moles of H2O from H2 = 1 mole H2 * (2 moles H2O / 2 moles H2) = 1 mole H2O

Moles of H2O from O2 = 1 mole O2 * (2 moles H2O / 1 mole O2) = 2 moles H2O

In this case, O2 produces more moles of water (2 moles) compared to H2 (1 mole). Therefore, H2 is the limiting reactant because it produces a smaller amount of water.

So, when the masses of reactants are the same, the limiting reactant is the one that produces fewer moles of products based on the stoichiometry of the balanced chemical equation.