How to identify a limitting reageant and solve problems by using POAC. please give an example ?

Dear Raunak,

Understanding limiting reagent problems, and being able to solve them, is essential for determining how much of each reactant is needed when performing a reaction, and will also tell you how much of each product will be formed in the reaction. The amount of products formed is determined by the limiting reagent.

Before solving a limiting reagent problem, you MUST first do a couple of things: you must write the balanced chemical reaction and you must determine the stoichiometry of the reaction. Instructions on how to do both of these things can be found under the Related Questions links to the left. 

Once you have written the balanced the reaction, and determined the stoichiometry, you must compare two (or more) calculations to determine which reagent is the limiting reagent and which one is in excess. 

The limiting reagent is the chemical which will effectively determine the amount of products that are formed. In other words, the limiting reagent is the chemical which will run out first as the reaction occurs and the reactants are consumed. The other reactants, the ones are are leftover when the limiting reagent runs out, are said to be in excess. 

Which chemical is limiting and which is in excess is ALWAYS a function of the stoichiometry and the number of moles, and NEVER the number of grams of the reagents. You must always compare the numbers of moles with the stoichiometry of the reaction. 

Let me explain how to solve the problem with an example.


Suppose 2.00g of NaCl reacts with 5.00g of AgNO₃ to form NaNO₃ and AgCl. First we must write the balanced equation, which is: 

NaCl + AgNO₃ ---> NaNO₃ + AgCl 

Now we must determine the stoichiometry, which is this case is very simple. The ratio of all species in the reaction is one-to-one. In other words, for one mole of NaCl, one mole of AgNO₃ reacts with it, and one mole of NaNO₃ and one mole of AgCl are formed. 

Now we must determine which reagent is limiting: the NaCl or the AgNO3. As I said, everything must be in terms of moles, and so we must convert the number of grams of each into moles: 

2.00 grams of NaCl ÷ 58.4425 grams/mole = 0.03422 moles NaCl 

5.00 grams of AgNO3 ÷ 169.987 grams/mole = 0.02944 moles AgNO₃

The stoichiometry of the reaction says that the number of moles of NaCl and AgNO₃ should be equal, but as you can see, in this case they are not the same. There are more moles of NaCl than AgNO₃. That means that AgNO₃ is the limiting reagent, and NaCl is in excess because there is more NaCl than can react with AgNO₃. That means that 0.02944 moles of NaCl will react with 0.02994 moles of AgNO₃, and 0.02944 moles of NaNO3 and 0.02944 moles of AgCl will be formed. 
From this we can say how much NaCl will be left one: the amount that we started with minus the amount that reacted: 

0.03422 - 0.02944 = 0.00478 moles of NaCl in excess. 

Note that NaCl is in excess even though there were fewer grams of NaCl than there were of AgNO₃! It's about MOLES and not grams. 

  Good Luck!!!!!!!!

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