LIMITING REAGENT (The Show Stopper)

Limiting reagent(L.R.) is a reagent which is consumed earlier in a chemical reaction.

Limiting reagent determines the amount of product formed in a given chemical reaction, the maximum amount of product formed is the amount formed by the L.R.

Identification of L.R.

e.g. Suppose you are given with 5 moles of A and 4 moles of B and they react according to the following reaction                            
                                                          4A  +  3B  ----------->  5C  

                                                           5         4  

Divide the given number of moles by respective stoichiometric coefficients

                                                5/4 = 1.25      4/3 = 1.33

Reactant having  lesser value of the two will be the L.R. Thus in the above reaction A is the L.R. and B is Excess Reagent (E.R.).

Moles of excess reagent left = Initial moles of E.R. - Coefficient of E.R./Coefficient of L.R.*  Initial  

                                                                                                                                             moles of L.R.

i.e.            Amount of B left = 4-3/4*5 = 4 - 3.75 = 0.25 mole

Moles of product formed     =  Initial moles of product - Coefficient of product/Coefficient of L.R.*  Initial   
                                                                                                                                                                         moles of L.R.

i.e.   Amount of C left = 5/4 *  5 = 25/4 = 6.25 mole

Remarks:- Use this special trick for faster calculations . You can solve any question of L.R. within 30-40 seconds once you have practiced this trick properly.