Comparing Units

[Gloria Qu 1K]

How do we know whether we compare the mols or grams when looking at limiting reactants?

[206363692]

When determining the limiting reactant, you always compare moles of the reactants, not grams. Grams of a substance aren't directly related to the stoichiometry unless converted to moles using the molar mass. Moles represent the number of particles (atoms, ions, molecules) participating in the reaction, therefore allowing us to directly apply the stoichiometric ratios. So when you find the limiting reactant, write the balanced chemical reaction, identify the mole ratios of the reactants from the coefficients, convert the given amounts of the reactants to mols, use the molar mass of each reactant to convert grams to mols (if given amounts are in grams), compare mols of reactants to the stoichiometric ratios, divide the mols of each reactant by its stoichiometric coefficient (from the balanced eqn), and the reactant with the smallest result will be your limiting reactant bc it is consumed first and stops the reaction.