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How do you solve and know which is the limiting reagent in chemical problems?

Limiting reagent is the chemical which dictates the chemical reaction in terms of whether the reaction takes place or not and how much reactants are consumed and products are formed. The simplest way to determine the limiting reagent in a chemical reaction is to write the well-balanced chemical equation of the reaction and determine the molar ratio in which reactants combine and products are formed. Then using the molar mass of each species, figure out the number of moles of each chemical and, using stoichiometry, find out the limiting reagent. 


For example, for cellular respiration, which takes place as per the following well-balanced chemical equation:


`C_6H_12O_6 + 6O_2 -> 6CO_2 + 6H_2O + ATP`


1 mole of glucose reacts with six moles of oxygen to form 6 moles of carbon dioxide and 6 moles of water and ATP molecules. The molar mass of glucose is 180 g, while that of oxygen is 32 g. If we are given 180 g of glucose, but only 96 g of oxygen, 1 mole of glucose will not be consumed. This is because 1 mole of glucose is given, but only 3 moles of oxygen are there and thus, using stoichiometry, only 0.5 mole glucose will react and we will get 3 moles each of CO2 and H2O. In this case, oxygen is the limiting reagent. 


If however, we were given 45 g of glucose and 96 g oxygen, then glucose would have been the limiting reagent. 

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