Will the limiting react always be the substance having the smallest mass?

Question

Naomi Aronson · Accepted Answer

Yes.

Indeed, the reason it is referred to as the limiting reactant is that it is the one that is consumed first in a chemical reaction, leaving the least amount of reactant in a chemical equation. On the other hand, excess is the largest amount. Limiting reactants and excess do not "exist" in a reaction involving a single product or reactant. I hope this is helpful.

Peyton Adams · Answer

Not necessarily.

The reagent with the fewer moles will be the limiting reagent. To find the limiting reagent, we first balance the chemical equation and convert all quantities of interest to moles. Next, we use stoichiometry to calculate the amount of product that each reactant could produce; the limiting reactant is the one that yields the lowest number of moles of product. Alternatively, we can compare the amount of moles of each reactant that is available; the limiting reactant is the one that is present in the lower number of moles. We cannot use the mass of each reagent present to make this determination because different elements have different molar masses, given in grams per mole. For example, if you have #120# grams of tin (Sn) and only #60# grams of lithium (Li), you might be inclined to think that because you have a lower mass quantity of lithium, it must be the limiting reactant. However, the molar mass of lithium is #~~7# g/mol, and the molar mass of tin is #~~119# g/mol. This means that you have #~~8.6# moles of lithium and only #~~1# mole of tin. Here is an example of calculating the limiting reagent in the reaction of #C_2H_3Br_3# (#76.4g#) with #O_2# (#49.1g#). Chemical equation in balance: #4C_2H_3Br_3+11O_2->8CO_2+6H_2O+6Br_2# Convert moles from mass quantities. #C_2H_3Br_3#: #266.72# g/mol #O_2#: #32# g/mol #76.4cancelg C_2H_3Br_3 xx (1mol)/(266.72cancelg)=0.286# #mol# #C_2H_3Br_3 # #49.1cancelg O_2 xx (1mol)/(32cancelg)=1.53# #mol# #O_2 # Therefore, #C_2H_3Br_3# is the limiting reagent, as it contributes the lower number of moles in the reaction. Notice that we began with a higher mass quantity of #C_2H_3Br_3#. You could also calculate the moles of #CO_2# produced by each reactant and compare using stoichiometry. In that method, whichever produces the lower amount of #CO_2# is the limiting reactant (still #C_2H_3Br_3#).

Natalie Anton · Answer

undefined No, the limiting reactant is not necessarily the substance with the smallest mass. It is determined by the stoichiometry of the reaction and the amounts of reactants present.